OneLinersCommand workbench
Verified diagnostic signature

Postfix deferred delivery because the next hop, recipient, DNS, capacity, or authentication failed.

Match output from postfix, understand the likely cause, then continue with sourced, read-only checks.

Confidence
high
Platforms
linux
Verified
2026-07-24
Recognized output

Signature matched locally

SASL authentication failed

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Likely causes

What usually produces this signal

  1. Remote MTA unavailable
  2. DNS failure
  3. Mailbox quota
  4. Relay authentication
Safe next checks

Collect evidence before changing the system

01

List the Postfix mail queue

postqueue -p

The output matches the expected target and exits without an error.

02

Show every Postfix log line for a queue ID

grep -F <postfix-queue-id> <postfix-log-file>

The output matches the expected target and exits without an error.

03

Resolve a hostname to its IP address

dig +short <domain>

The output matches the expected target and exits without an error.