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SMTP Error 421 4.7.28: Rate limited: unusual rate of unsolicited mail

Warning↻ Retryable · exponential backoffNetwork

TL;DR · 10-second read

Gmail throttles your domain or IP because of spam complaints or a sudden volume spike. Slow down, warm up new IPs gradually and keep the spam rate below 0.3% in Postmaster Tools.

Identify: does your log match?

What you see · exact log / response

421-4.7.28 Gmail has detected an unusual rate of unsolicited mail from your
421-4.7.28 domain. To protect our users from spam, mail sent from your domain has
421 4.7.28 been temporarily rate limited. - gsmtp

Fix

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Last verified 2026-10-21 · Source: RFC 5321 reply codes + RFC 3463 enhanced status codes (IANA registry) · Official docs ↗ · ~2,400 searches/mo