SMTP: 421 4.7.28 Our system has detected an unusual rate of unsolicited mail

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.

# How much is being deferred, and to which domain?
grep '4.7.28' /var/log/mail.log | wc -l
postqueue -p | grep -c gmail

# Slow down delivery to Gmail rather than retrying harder (Postfix)
#   /etc/postfix/transport:  gmail.com  slow:
#   master.cf: slow unix - - n - 2 smtp
#   main.cf:   slow_destination_rate_delay = 2s
postmap /etc/postfix/transport && postfix reload

# Check spam rate and IP reputation in Google Postmaster Tools
# Keep the user-reported spam rate below 0.3%

# Warm up a new IP gradually over a few weeks

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.

1. 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.
2. Test the SMTP conversation manually with swaks --to [email protected] --server smtp.example.com --tls. It shows each command and response.
3. Verify SPF, DKIM and DMARC records with dig TXT example.com, dig TXT selector._domainkey.example.com and dig TXT _dmarc.example.com.
4. 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.
5. 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

• swaks
• dig TXT
• openssl s_client -starttls smtp
• postfix mail logs
• DMARC aggregate reports

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

📖 support.google.com 📖 support.google.com

Related Email errors

• DKIM: Signature verification failedThe DKIM signature on the email does not match. The DNS public key record may be wrong, or…
• DMARC: dmarc=fail (p=reject) in Authentication-ResultsSPF or DKIM passed on its own but neither aligned with the domain in the From header, and…
• Postfix: 450 4.1.8 Sender address rejected: Domain not foundThe receiving Postfix has reject_unknown_sender_domain enabled and could not find an A or MX…
• Postfix: connect to mx1.example.net[203.0.113.25]:25: Connection timed outYour server never got a TCP connection to the recipient's mail server on port 25, so the…
• Postfix: SASL authentication failure: No worthy mechs foundPostfix tried to log in to the relay and Cyrus SASL could not find a mechanism the relay…
• SMTP: 421 Too many connectionsThe mail server is rate limiting connections. Too many simultaneous connections from the same…
• SMTP: 451 4.7.1 Greylisted, please try again laterThe receiving server deliberately rejected a first delivery attempt from an unknown sender…
• SMTP: 530 5.7.0 Must issue a STARTTLS command firstThe server requires an encrypted session before it will accept AUTH or MAIL FROM, and the…

See all 20 Email errors →

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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.