SMTP: 451 4.7.1 Greylisted, please try again later
The receiving server deliberately rejected a first delivery attempt from an unknown sender triple and expects a retry, which real mail servers do and most spam software does not. It is a temporary failure, so the only bug is in senders that treat any 4xx as final and drop the message.
Quick fix
Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.
# 4xx is retryable, 5xx is not: the client must queue rather than fail
# Postfix queue behaviour for the deferred message
postqueue -p | head
postcat -q <QUEUEID> | head -30
mailq
# Force an immediate retry once, to confirm the second attempt is accepted
postqueue -f
# Retry cadence, which decides how long the delay lasts in practice
postconf minimal_backoff_time maximal_backoff_time maximal_queue_lifetime
# Application senders: do not swallow a 4xx into a failed job
# python: smtplib raises SMTPDataError, .smtp_code starts with 4 means retry later
# A few minutes of delay is normal. Hours means the triple keeps changing,
# which happens when outbound traffic leaves through a rotating pool of IPs.
# Sending from a fixed relay address fixes it properly.How to diagnose Email errors
SMTP errors follow a strict convention: a 4xx code is temporary and will be retried, a 5xx is permanent and will not. Beyond delivery mechanics, most modern email problems are authentication problems (SPF, DKIM and DMARC alignment) which cause silent filtering into spam rather than an explicit bounce, and therefore need to be checked proactively.
If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.
- Read the full bounce message. The enhanced status code (for example
5.7.1) is far more specific than the three-digit code and usually names the exact policy that rejected the message. - Test the SMTP conversation manually with
swaks --to [email protected] --server smtp.example.com --tls. It shows each command and response. - Verify SPF, DKIM and DMARC records with
dig TXT example.com,dig TXT selector._domainkey.example.comanddig TXT _dmarc.example.com. - Check DMARC alignment, not just presence: the domain in the
From:header must align with the SPF or DKIM domain, or DMARC fails even when both pass individually. - Confirm the sending IP is not on a blocklist and has valid reverse DNS. Missing PTR records cause rejection by many large providers.
Tools worth reaching for
swaksdig TXTopenssl s_client -starttls smtppostfix mail logsDMARC aggregate reports
Authoritative references
Primary documentation for this error, worth reading before applying any fix in production.
Related Email errors
- DKIM: Signature verification failedThe DKIM signature on the email does not match. The DNS public key record may be wrong, or…
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Something missing or wrong?
This entry is maintained by hand. If the fix is out of date, incomplete, or you have a better one, email a correction and it will be reviewed.