Security / Services & Applications

Configure Dovecot IMAP with TLS and secure authentication

Install Dovecot, set mailbox location, require TLS, protect certificates, choose authentication mechanisms, constrain listeners, and test IMAPS and STARTTLS.

75 min8 stepsChanges system stateRevision 2

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#dovecot #imap #tls #authentication #maildir

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Goal

Provide authenticated mailbox access without plaintext credentials or ambiguous storage ownership.

Supported environments

Dovecot CE 2.4

Prerequisites

Authoritative DNS and stable identity Control forward and reverse DNS, choose the canonical mail hostname and domains, and verify the host is not behind an outbound SMTP-blocking network.hostname -f; getent ahosts $(hostname -f); dig +short -x {{publicIp}}
Recovery and queue safety Keep console access, back up active configuration, and record queue state before changing routing, authentication, filters, or mailbox delivery.
Abuse and deliverability plan Define relay boundaries, account owners, rate limits, bounce handling, postmaster address, monitoring, and how compromised credentials will be revoked.

Operating boundary

OneLiners never runs these steps or stores secrets. Review placeholders, versions, current state, and change-control requirements before using a command.

Full guide

What you will build

System

A production Dovecot implementation of “Configure Dovecot IMAP with TLS and secure authentication” with explicit SMTP/IMAP boundaries, identity, durable state and observable delivery evidence.
A read-only baseline, staged candidate and end-to-end canary covering DNS, TLS, authentication, recipient or route policy, queue/mailbox state and logs.
A security and rollback procedure that avoids open relay, backscatter, plaintext credentials, world-readable keys/mail and unreviewed bulk queue actions.

Observable outcome

Provide authenticated mailbox access without plaintext credentials or ambiguous storage ownership. The intended success path is visible in native status, protocol transcript, headers or mailbox evidence.
Expected unauthorized relay, unknown-recipient, wrong-sender, invalid-authentication or wrong-certificate tests fail at the declared boundary without harming unrelated mail.

Architecture

How the parts fit together

Dovecot exposes IMAP over a certificate-authenticated TLS endpoint, authenticates against the chosen identity backend and maps the user to an explicitly owned Maildir. Plaintext authentication is permitted only inside TLS. Protocol, cipher, certificate, login and mailbox-access evidence are checked separately. The MTA delivery path remains unchanged until Dovecot can read the existing mailbox safely.

TLS listenerAuthenticates the mail hostname before any password exchange.

Authentication backendValidates user identity under explicit password/hash policy.

Mailbox namespaceMaps identity to private owned Maildir content.

Protocol logs/testsProve negotiation, login and message access without publishing secrets.

Inventory hostname/DNS, listeners, certificates, identities, recipient/relay maps, queue/mailbox state, logs and a recoverable configuration/data baseline.
Prepare the imap candidate with official Dovecot syntax, protected secrets and one explicit service/data boundary at a time.
Run native configuration validation and immediate read-only checkpoint before reload or acceptance of new messages.
Use controlled local and external canaries plus an expected-denial test, preserve message identifiers/headers, and retain the prior configuration for rollback.

Assumptions

The public mail hostname, forward/reverse DNS ownership, supported package version and responsibility for ports 25/587/993 are known.
A maintenance or canary window, console recovery and current protected backup of configuration, keys, identity mappings and relevant mailbox/queue data exist.
Test sender and recipient addresses are owned by the operator; no unsolicited or third-party traffic is generated during verification.
Secrets and private keys are delivered outside shell history and have narrowly defined service ownership and rotation plans.

Key concepts

envelope identity: SMTP sender and recipient used for routing, delivery status and SPF; it is distinct from visible message headers.
open relay: An MTA that accepts unauthorized third-party sender-to-recipient forwarding, enabling abuse and reputation damage.
backscatter: A later non-delivery report sent to a forged sender because an invalid recipient was accepted instead of rejected during SMTP.
alignment: DMARC relationship between visible From domain and authenticated SPF or DKIM domain.
Maildir: A mailbox format storing messages as individual files in cur/new/tmp directories with strict ownership.

Before you copy

Values used in this guide

{{mailHost}}

Operator-reviewed value used by this tutorial for mailHost; derive it from the target environment and never from untrusted request data.

Example: mail.example.com

{{mailRoot}}

Operator-reviewed value used by this tutorial for mailRoot; derive it from the target environment and never from untrusted request data.

Example: /srv/vmail

{{protectedSecret}}

Operator-reviewed value used by this tutorial for protectedSecret; derive it from the target environment and never from untrusted request data.

Example: secret-manager-reference

{{user}}

Operator-reviewed value used by this tutorial for user; derive it from the target environment and never from untrusted request data.

Example: alice@example.com

Security and production boundaries

Never weaken relay/recipient checks, publish private DKIM/TLS keys, enable plaintext authentication, or expose web/IMAP administration broadly to make a test pass.
Mail content, credentials, queues, logs and backups contain personal or confidential data; restrict, encrypt, minimize and retain them under an explicit policy.
Use dedicated service identities and root-readable credential maps; verify logs and configuration output are redacted before sharing.

Stop before continuing if

Stop if a test permits unauthorized relay, accepts unknown recipients for later bounce, sends credentials without verified TLS, or exposes private keys/mailbox files.
Do not continue when hostname/PTR ownership, package/config version, recipient map, route, queue, mailbox ownership or rollback backup is ambiguous.
Immediately quarantine the candidate when unrelated mail is rerouted, deleted, multiply delivered, signed by the wrong domain, or inaccessible after reload.

instruction

Inventory users, mailbox format, ports, and existing auth

read-only

Record whether users are system or virtual, storage path, UID/GID ownership, current listeners, certificate identity, and migration requirements.

Why this step matters

Inventory users, mailbox format, ports, and existing auth is a separate checkpoint because it establishes one auditable part of provide authenticated mailbox access without plaintext credentials or ambiguous storage ownership. Performing it independently keeps a failed prerequisite, syntax error, or compatibility mismatch from being hidden by later actions.

What to understand

Record whether users are system or virtual, storage path, UID/GID ownership, current listeners, certificate identity, and migration requirements. Read the complete command or configuration before execution, replace the documented placeholders, and compare the target host with the supported environments listed at the beginning of this tutorial.

Capture the before-state and the exact output in the change record. If this step modifies a file or service, validate the candidate with its native checker before any reload; a successful process exit alone does not prove that the application uses the intended value.

Use a representative staging or canary host first. Repeat the stated checkpoint immediately, because proceeding with ambiguous evidence makes the later end-to-end verification and rollback materially harder.

System changes

This step may affect the files, packages, identities, services, or runtime policy named in “Inventory users, mailbox format, ports, and existing auth”. The scope must remain limited to the selected application and supported host.

Syntax explained

doveconf: Invokes the principal tool or configuration operation for “Inventory users, mailbox format, ports, and existing auth”; confirm that it resolves to the expected packaged executable or file before using elevated privileges.
{{…}}: Marks an operator-supplied value. Replace every placeholder deliberately; do not paste untrusted text or leave braces in the production command.
|: Passes standard output to the next read-only inspection stage; review each stage separately if the combined result is surprising.

Command

Fill variables0/1 ready

Values stay on this page and are never sent or saved.

Mail Root{{mailRoot}}Operator-reviewed value used by this tutorial for mailRoot; derive it from the target environment and never from untrusted request data.

sudo doveconf -n 2>/dev/null || true; sudo ss -lntp | grep -E ':(143|993)' || true; sudo find {{mailRoot}} -maxdepth 2 -printf '%m %u:%g %p\n' 2>/dev/null | head

Example output / evidence

Identity, storage, and listener baseline are recorded.

Checkpoint: Checkpoint: Inventory users, mailbox format, ports, and existing auth

Continue whenIdentity, storage, and listener baseline are recorded. Treat this concrete evidence as the minimum success signal and also confirm that unrelated services and the previous healthy path remain unchanged.

Stop whenStop before the next step when doveconf reports an error, the output differs materially from the example, an unexpected package/service/path is affected, or the required before-state and rollback artifact are missing.

If this step fails

The command for “Inventory users, mailbox format, ports, and existing auth” fails, produces no useful evidence, or the expected service/configuration state is absent.

Likely causeA placeholder targets the wrong environment, the installed version uses another path or unit name, permissions are insufficient, or an earlier prerequisite was not satisfied.

Safe checks

command -v doveconf 2>/dev/null || true
systemctl --failed --no-pager 2>/dev/null || true
journalctl -p warning -n 50 --no-pager 2>/dev/null || true

ResolutionDo not improvise a privileged workaround. Re-read the environment and variable table, inspect the official source for the installed version, restore the candidate file if necessary, and repeat only this checkpoint.

Security notes

Keep secrets out of shell history, process arguments, screenshots, logs, and tutorial placeholders. Elevate only the narrow command that requires it and preserve package signature and TLS verification.

Alternatives

When the host layout or software version differs, use the vendor-supported equivalent in a disposable staging environment and document the mapping before touching production.

Stop conditions

Stop if the action broadens network exposure, permissions, package sources, writable paths, or service privileges beyond the tutorial's declared architecture.

command

Install Dovecot IMAP packages

caution

Use supported distribution packages for core, IMAP, and the selected authentication/storage backend.

Why this step matters

Install Dovecot IMAP packages is a separate checkpoint because it establishes one auditable part of provide authenticated mailbox access without plaintext credentials or ambiguous storage ownership. Performing it independently keeps a failed prerequisite, syntax error, or compatibility mismatch from being hidden by later actions.

What to understand

Use supported distribution packages for core, IMAP, and the selected authentication/storage backend. Read the complete command or configuration before execution, replace the documented placeholders, and compare the target host with the supported environments listed at the beginning of this tutorial.

System changes

This step may affect the files, packages, identities, services, or runtime policy named in “Install Dovecot IMAP packages”. The scope must remain limited to the selected application and supported host.

Syntax explained

apt: Invokes the principal tool or configuration operation for “Install Dovecot IMAP packages”; confirm that it resolves to the expected packaged executable or file before using elevated privileges.
&&: Runs the following operation only when the previous command succeeds, preventing a reload or state transition after a failed validation.

Command

sudo apt update && sudo apt install --yes dovecot-core dovecot-imapd

Example output / evidence

Dovecot packages install without starting an unintended public plaintext service.

Checkpoint: Checkpoint: Install Dovecot IMAP packages

Continue whenDovecot packages install without starting an unintended public plaintext service. Treat this concrete evidence as the minimum success signal and also confirm that unrelated services and the previous healthy path remain unchanged.

Stop whenStop before the next step when apt reports an error, the output differs materially from the example, an unexpected package/service/path is affected, or the required before-state and rollback artifact are missing.

If this step fails

The command for “Install Dovecot IMAP packages” fails, produces no useful evidence, or the expected service/configuration state is absent.

Likely causeA placeholder targets the wrong environment, the installed version uses another path or unit name, permissions are insufficient, or an earlier prerequisite was not satisfied.

Safe checks

command -v apt 2>/dev/null || true
systemctl --failed --no-pager 2>/dev/null || true
journalctl -p warning -n 50 --no-pager 2>/dev/null || true

Security notes

Keep secrets out of shell history, process arguments, screenshots, logs, and tutorial placeholders. Elevate only the narrow command that requires it and preserve package signature and TLS verification.

Alternatives

When the host layout or software version differs, use the vendor-supported equivalent in a disposable staging environment and document the mapping before touching production.

Stop conditions

Stop if the action broadens network exposure, permissions, package sources, writable paths, or service privileges beyond the tutorial's declared architecture.

config

Set mailbox format and ownership explicitly

caution

Configure Maildir or the selected storage, namespaces, first-valid UID, and private indexes; migrate no existing mail until a tested copy exists.

Why this step matters

Set mailbox format and ownership explicitly is a separate checkpoint because it establishes one auditable part of provide authenticated mailbox access without plaintext credentials or ambiguous storage ownership. Performing it independently keeps a failed prerequisite, syntax error, or compatibility mismatch from being hidden by later actions.

What to understand

Configure Maildir or the selected storage, namespaces, first-valid UID, and private indexes; migrate no existing mail until a tested copy exists. Read the complete command or configuration before execution, replace the documented placeholders, and compare the target host with the supported environments listed at the beginning of this tutorial.

System changes

This step may affect the files, packages, identities, services, or runtime policy named in “Set mailbox format and ownership explicitly”. The scope must remain limited to the selected application and supported host.

Syntax explained

sudoedit: Invokes the principal tool or configuration operation for “Set mailbox format and ownership explicitly”; confirm that it resolves to the expected packaged executable or file before using elevated privileges.
|: Passes standard output to the next read-only inspection stage; review each stage separately if the combined result is surprising.
/etc/dovecot/conf.d/10-mail.conf: Is the persistent configuration boundary changed by this step. Keep an attributable backup and validate the consumer before reload.

File /etc/dovecot/conf.d/10-mail.conf

Configuration

sudoedit /etc/dovecot/conf.d/10-mail.conf; sudo doveconf -n mail_driver mail_path 2>/dev/null || sudo doveconf -n mail_location

Example output / evidence

Dovecot resolves the intended mailbox and index paths.

Checkpoint: Checkpoint: Set mailbox format and ownership explicitly

Continue whenDovecot resolves the intended mailbox and index paths. Treat this concrete evidence as the minimum success signal and also confirm that unrelated services and the previous healthy path remain unchanged.

Stop whenStop before the next step when sudoedit reports an error, the output differs materially from the example, an unexpected package/service/path is affected, or the required before-state and rollback artifact are missing.

If this step fails

The command for “Set mailbox format and ownership explicitly” fails, produces no useful evidence, or the expected service/configuration state is absent.

Likely causeA placeholder targets the wrong environment, the installed version uses another path or unit name, permissions are insufficient, or an earlier prerequisite was not satisfied.

Safe checks

command -v sudoedit 2>/dev/null || true
systemctl --failed --no-pager 2>/dev/null || true
journalctl -p warning -n 50 --no-pager 2>/dev/null || true

Security notes

Keep secrets out of shell history, process arguments, screenshots, logs, and tutorial placeholders. Elevate only the narrow command that requires it and preserve package signature and TLS verification.

Alternatives

When the host layout or software version differs, use the vendor-supported equivalent in a disposable staging environment and document the mapping before touching production.

Stop conditions

Stop if the action broadens network exposure, permissions, package sources, writable paths, or service privileges beyond the tutorial's declared architecture.

config

Configure a trusted IMAP certificate

caution

Use a SAN matching the client hostname, complete chain, protected key, ssl required, and current protocol/cipher policy.

Why this step matters

Configure a trusted IMAP certificate is a separate checkpoint because it establishes one auditable part of provide authenticated mailbox access without plaintext credentials or ambiguous storage ownership. Performing it independently keeps a failed prerequisite, syntax error, or compatibility mismatch from being hidden by later actions.

What to understand

Use a SAN matching the client hostname, complete chain, protected key, ssl required, and current protocol/cipher policy. Read the complete command or configuration before execution, replace the documented placeholders, and compare the target host with the supported environments listed at the beginning of this tutorial.

System changes

This step may affect the files, packages, identities, services, or runtime policy named in “Configure a trusted IMAP certificate”. The scope must remain limited to the selected application and supported host.

Syntax explained

sudoedit: Invokes the principal tool or configuration operation for “Configure a trusted IMAP certificate”; confirm that it resolves to the expected packaged executable or file before using elevated privileges.
|: Passes standard output to the next read-only inspection stage; review each stage separately if the combined result is surprising.
/etc/dovecot/conf.d/10-ssl.conf: Is the persistent configuration boundary changed by this step. Keep an attributable backup and validate the consumer before reload.

File /etc/dovecot/conf.d/10-ssl.conf

Configuration

sudoedit /etc/dovecot/conf.d/10-ssl.conf; sudo doveconf -n | grep -E 'ssl|ssl_server_cert_file|ssl_server_key_file'

Example output / evidence

Dovecot requires TLS and points to the approved identity.

Checkpoint: Checkpoint: Configure a trusted IMAP certificate

Continue whenDovecot requires TLS and points to the approved identity. Treat this concrete evidence as the minimum success signal and also confirm that unrelated services and the previous healthy path remain unchanged.

If this step fails

The command for “Configure a trusted IMAP certificate” fails, produces no useful evidence, or the expected service/configuration state is absent.

Likely causeA placeholder targets the wrong environment, the installed version uses another path or unit name, permissions are insufficient, or an earlier prerequisite was not satisfied.

Safe checks

command -v sudoedit 2>/dev/null || true
systemctl --failed --no-pager 2>/dev/null || true
journalctl -p warning -n 50 --no-pager 2>/dev/null || true

Security notes

Keep secrets out of shell history, process arguments, screenshots, logs, and tutorial placeholders. Elevate only the narrow command that requires it and preserve package signature and TLS verification.

Alternatives

When the host layout or software version differs, use the vendor-supported equivalent in a disposable staging environment and document the mapping before touching production.

Stop conditions

Stop if the action broadens network exposure, permissions, package sources, writable paths, or service privileges beyond the tutorial's declared architecture.

config

Limit authentication mechanisms

caution

Use PLAIN only inside required TLS, disable plaintext auth before TLS, choose system or virtual passwd database, and remove unused mechanisms.

Why this step matters

Limit authentication mechanisms is a separate checkpoint because it establishes one auditable part of provide authenticated mailbox access without plaintext credentials or ambiguous storage ownership. Performing it independently keeps a failed prerequisite, syntax error, or compatibility mismatch from being hidden by later actions.

What to understand

Use PLAIN only inside required TLS, disable plaintext auth before TLS, choose system or virtual passwd database, and remove unused mechanisms. Read the complete command or configuration before execution, replace the documented placeholders, and compare the target host with the supported environments listed at the beginning of this tutorial.

System changes

This step may affect the files, packages, identities, services, or runtime policy named in “Limit authentication mechanisms”. The scope must remain limited to the selected application and supported host.

Syntax explained

sudoedit: Invokes the principal tool or configuration operation for “Limit authentication mechanisms”; confirm that it resolves to the expected packaged executable or file before using elevated privileges.
|: Passes standard output to the next read-only inspection stage; review each stage separately if the combined result is surprising.
/etc/dovecot/conf.d/10-auth.conf: Is the persistent configuration boundary changed by this step. Keep an attributable backup and validate the consumer before reload.

File /etc/dovecot/conf.d/10-auth.conf

Configuration

sudoedit /etc/dovecot/conf.d/10-auth.conf; sudo doveconf -n | grep -E 'auth_mechanisms|auth_allow_cleartext|passdb|userdb'

Example output / evidence

Authentication backend and TLS precondition are explicit.

Checkpoint: Checkpoint: Limit authentication mechanisms

Continue whenAuthentication backend and TLS precondition are explicit. Treat this concrete evidence as the minimum success signal and also confirm that unrelated services and the previous healthy path remain unchanged.

If this step fails

The command for “Limit authentication mechanisms” fails, produces no useful evidence, or the expected service/configuration state is absent.

Likely causeA placeholder targets the wrong environment, the installed version uses another path or unit name, permissions are insufficient, or an earlier prerequisite was not satisfied.

Safe checks

command -v sudoedit 2>/dev/null || true
systemctl --failed --no-pager 2>/dev/null || true
journalctl -p warning -n 50 --no-pager 2>/dev/null || true

Security notes

Keep secrets out of shell history, process arguments, screenshots, logs, and tutorial placeholders. Elevate only the narrow command that requires it and preserve package signature and TLS verification.

Alternatives

When the host layout or software version differs, use the vendor-supported equivalent in a disposable staging environment and document the mapping before touching production.

Stop conditions

Stop if the action broadens network exposure, permissions, package sources, writable paths, or service privileges beyond the tutorial's declared architecture.

config

Constrain IMAP listeners and connection limits

caution

Enable 993 and optionally STARTTLS on 143, bind only intended addresses, set process/client limits, and apply firewall policy.

Why this step matters

Constrain IMAP listeners and connection limits is a separate checkpoint because it establishes one auditable part of provide authenticated mailbox access without plaintext credentials or ambiguous storage ownership. Performing it independently keeps a failed prerequisite, syntax error, or compatibility mismatch from being hidden by later actions.

What to understand

Enable 993 and optionally STARTTLS on 143, bind only intended addresses, set process/client limits, and apply firewall policy. Read the complete command or configuration before execution, replace the documented placeholders, and compare the target host with the supported environments listed at the beginning of this tutorial.

System changes

This step may affect the files, packages, identities, services, or runtime policy named in “Constrain IMAP listeners and connection limits”. The scope must remain limited to the selected application and supported host.

Syntax explained

sudoedit: Invokes the principal tool or configuration operation for “Constrain IMAP listeners and connection limits”; confirm that it resolves to the expected packaged executable or file before using elevated privileges.
configuration file: Is the persistent configuration boundary changed by this step. Keep an attributable backup and validate the consumer before reload.

Configuration

sudoedit /etc/dovecot/conf.d/10-master.conf; sudo ufw allow 993/tcp; sudo ufw allow 143/tcp; sudo ufw status numbered

Example output / evidence

Only approved IMAP ports and addresses are exposed.

Checkpoint: Checkpoint: Constrain IMAP listeners and connection limits

Continue whenOnly approved IMAP ports and addresses are exposed. Treat this concrete evidence as the minimum success signal and also confirm that unrelated services and the previous healthy path remain unchanged.

If this step fails

The command for “Constrain IMAP listeners and connection limits” fails, produces no useful evidence, or the expected service/configuration state is absent.

Likely causeA placeholder targets the wrong environment, the installed version uses another path or unit name, permissions are insufficient, or an earlier prerequisite was not satisfied.

Safe checks

command -v sudoedit 2>/dev/null || true
systemctl --failed --no-pager 2>/dev/null || true
journalctl -p warning -n 50 --no-pager 2>/dev/null || true

Security notes

Keep secrets out of shell history, process arguments, screenshots, logs, and tutorial placeholders. Elevate only the narrow command that requires it and preserve package signature and TLS verification.

Alternatives

When the host layout or software version differs, use the vendor-supported equivalent in a disposable staging environment and document the mapping before touching production.

Stop conditions

Stop if the action broadens network exposure, permissions, package sources, writable paths, or service privileges beyond the tutorial's declared architecture.

verification

Validate, restart, and inspect logs

caution

Run doveconf, restart, inspect listeners, certificate, and recent authentication/storage errors.

Why this step matters

Validate, restart, and inspect logs is a separate checkpoint because it establishes one auditable part of provide authenticated mailbox access without plaintext credentials or ambiguous storage ownership. Performing it independently keeps a failed prerequisite, syntax error, or compatibility mismatch from being hidden by later actions.

What to understand

Run doveconf, restart, inspect listeners, certificate, and recent authentication/storage errors. Read the complete command or configuration before execution, replace the documented placeholders, and compare the target host with the supported environments listed at the beginning of this tutorial.

System changes

This step may affect the files, packages, identities, services, or runtime policy named in “Validate, restart, and inspect logs”. The scope must remain limited to the selected application and supported host.

Syntax explained

doveconf: Invokes the principal tool or configuration operation for “Validate, restart, and inspect logs”; confirm that it resolves to the expected packaged executable or file before using elevated privileges.
|: Passes standard output to the next read-only inspection stage; review each stage separately if the combined result is surprising.

Command

sudo doveconf -n; sudo systemctl restart dovecot; sudo ss -lntp | grep -E ':(143|993)'; sudo journalctl -u dovecot -n 100 --no-pager

Example output / evidence

Dovecot starts and owns intended listeners without config errors.

Checkpoint: Checkpoint: Validate, restart, and inspect logs

Continue whenDovecot starts and owns intended listeners without config errors. Treat this concrete evidence as the minimum success signal and also confirm that unrelated services and the previous healthy path remain unchanged.

If this step fails

The command for “Validate, restart, and inspect logs” fails, produces no useful evidence, or the expected service/configuration state is absent.

Likely causeA placeholder targets the wrong environment, the installed version uses another path or unit name, permissions are insufficient, or an earlier prerequisite was not satisfied.

Safe checks

command -v doveconf 2>/dev/null || true
systemctl --failed --no-pager 2>/dev/null || true
journalctl -p warning -n 50 --no-pager 2>/dev/null || true

Security notes

Keep secrets out of shell history, process arguments, screenshots, logs, and tutorial placeholders. Elevate only the narrow command that requires it and preserve package signature and TLS verification.

Alternatives

When the host layout or software version differs, use the vendor-supported equivalent in a disposable staging environment and document the mapping before touching production.

Stop conditions

Stop if the action broadens network exposure, permissions, package sources, writable paths, or service privileges beyond the tutorial's declared architecture.

verification

Test TLS, login, mailbox listing, and plaintext denial

read-only

Verify certificate hostname, authenticate a test user, list/select a mailbox, test wrong credentials, and prove LOGIN is unavailable before STARTTLS.

Why this step matters

Test TLS, login, mailbox listing, and plaintext denial is a separate checkpoint because it establishes one auditable part of provide authenticated mailbox access without plaintext credentials or ambiguous storage ownership. Performing it independently keeps a failed prerequisite, syntax error, or compatibility mismatch from being hidden by later actions.

What to understand

Verify certificate hostname, authenticate a test user, list/select a mailbox, test wrong credentials, and prove LOGIN is unavailable before STARTTLS. Read the complete command or configuration before execution, replace the documented placeholders, and compare the target host with the supported environments listed at the beginning of this tutorial.

System changes

This step may affect the files, packages, identities, services, or runtime policy named in “Test TLS, login, mailbox listing, and plaintext denial”. The scope must remain limited to the selected application and supported host.

Syntax explained

openssl: Invokes the principal tool or configuration operation for “Test TLS, login, mailbox listing, and plaintext denial”; confirm that it resolves to the expected packaged executable or file before using elevated privileges.
{{…}}: Marks an operator-supplied value. Replace every placeholder deliberately; do not paste untrusted text or leave braces in the production command.

Command

Fill variables0/3 ready

Values stay on this page and are never sent or saved.

Mail Host{{mailHost}}Operator-reviewed value used by this tutorial for mailHost; derive it from the target environment and never from untrusted request data.User{{user}}Operator-reviewed value used by this tutorial for user; derive it from the target environment and never from untrusted request data.Protected Secret{{protectedSecret}}Operator-reviewed value used by this tutorial for protectedSecret; derive it from the target environment and never from untrusted request data.

openssl s_client -connect {{mailHost}}:993 -servername {{mailHost}} -verify_return_error -crlf </dev/null; doveadm auth test {{user}} '{{protectedSecret}}'; doveadm mailbox list -u {{user}}

Example output / evidence

Certificate and valid auth succeed; invalid and plaintext paths fail.

Checkpoint: Checkpoint: Test TLS, login, mailbox listing, and plaintext denial

Continue whenCertificate and valid auth succeed; invalid and plaintext paths fail. Treat this concrete evidence as the minimum success signal and also confirm that unrelated services and the previous healthy path remain unchanged.

Stop whenStop before the next step when openssl reports an error, the output differs materially from the example, an unexpected package/service/path is affected, or the required before-state and rollback artifact are missing.

If this step fails

The command for “Test TLS, login, mailbox listing, and plaintext denial” fails, produces no useful evidence, or the expected service/configuration state is absent.

Likely causeA placeholder targets the wrong environment, the installed version uses another path or unit name, permissions are insufficient, or an earlier prerequisite was not satisfied.

Safe checks

command -v openssl 2>/dev/null || true
systemctl --failed --no-pager 2>/dev/null || true
journalctl -p warning -n 50 --no-pager 2>/dev/null || true

Security notes

Keep secrets out of shell history, process arguments, screenshots, logs, and tutorial placeholders. Elevate only the narrow command that requires it and preserve package signature and TLS verification.

Alternatives

When the host layout or software version differs, use the vendor-supported equivalent in a disposable staging environment and document the mapping before touching production.

Stop conditions

Stop if the action broadens network exposure, permissions, package sources, writable paths, or service privileges beyond the tutorial's declared architecture.

Finish line

Verification checklist

TLS identityopenssl s_client -connect {{mailHost}}:993 -servername {{mailHost}} -verify_return_error </dev/nullCertificate chain and hostname validate.

Authentication and storagesudo doveadm auth test {{user}} '{{protectedSecret}}' && sudo doveadm mailbox status -u {{user}} messages INBOXAuthentication succeeds and INBOX metadata is readable.

Recovery guidance

Common problems and safe checks

A message remains queued or the remote server returns a temporary 4xx response.

Likely causeDNS/MX resolution, routing, network reachability, TLS negotiation, remote greylisting/rate limits, reputation or recipient policy prevents delivery.

Safe checks

exim -bp 2>/dev/null || postqueue -p
dig +short MX example.net
journalctl -u exim4 -u postfix --since '-30 min' --no-pager

ResolutionInspect one message's recorded diagnostic and route first, fix the evidenced dependency, then retry only that message; do not force an undiagnosed bulk queue.

SMTP accepts a recipient, but the mailbox is absent, unreadable or never visible over IMAP.

Likely causeRecipient maps, LMTP/LDA route, virtual UID/GID, Maildir path, quota, Dovecot namespace or filesystem permissions disagree.

Safe checks

doveconf -n
doveadm user user@example.com
doveadm mailbox list -u user@example.com

ResolutionTrace the exact address through recipient validation, delivery socket and resolved mailbox path; correct the narrow mapping/ownership mismatch before accepting more mail.

Authentication or TLS fails despite apparently correct credentials and certificate files.

Likely causeThe listener uses another configuration, hostname/SAN does not match, chain/key permissions are wrong, authentication is attempted without TLS, or the identity backend maps another username.

Safe checks

openssl s_client -connect mail.example.com:587 -starttls smtp -servername mail.example.com </dev/null
openssl s_client -connect mail.example.com:993 -servername mail.example.com </dev/null
journalctl -u dovecot -u postfix -u exim4 -p warning -n 100 --no-pager

ResolutionVerify the advertised hostname, chain, listener and mapped user independently; never disable certificate verification or enable plaintext authentication to bypass the failure.

After the procedure

Alternatives and next steps

Consider these alternatives

Use a reputable managed mail transport or mailbox provider when staffing, deliverability, abuse response, DNS reputation and 24/7 queue operations cannot be sustained.
Separate inbound, outbound, submission and mailbox roles across hosts only when TLS identity, routing, backups and observability remain end-to-end testable.

Operate it safely

Alert on queue age/depth, delivery deferrals, authentication failures, TLS/certificate expiry, disk/quota pressure, DKIM/DMARC failures and backup restore age.
Review relay/recipient maps, dormant users, supported versions, DNS policy and key/certificate rotation at least every 90 days.
Practice one-message queue diagnosis and an isolated mailbox/config restore so incident steps are proven before a real outage.

Reference

Frequently asked questions

Why is one successful test email insufficient?

It may bypass the actual external DNS, authentication, recipient, DKIM, queue retry or IMAP path. Test each boundary and preserve protocol/header evidence.

Can TLS be tested by checking that encryption is present?

No. Verify the chain and hostname and ensure authentication is unavailable before STARTTLS; encryption to an unverified endpoint is not server authentication.

Should a stuck queue be forced immediately?

No. Read one message's diagnostic and fix the dependency first. Bulk retries can amplify rate limits, duplicates or reputation damage.

Recovery

Rollback

Restore prior Dovecot configuration and listener policy without modifying mailbox files.

Restore /etc/dovecot, run doveconf -n, and restart.
Restore prior firewall rules only after confirming intended client ports.
Do not move or delete mail storage during authentication rollback.

Evidence

Sources and review

Verified 2026-07-25Review due 2026-10-23

Dovecot SSL configurationofficial Dovecot virtual usersofficial Dovecot SASL authentication for Postfixofficial