Bounce codes. What each SMTP bounce code means, and whose problem it is.
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550 5.7.515: Microsoft wants SPF, DKIM, and DMARC before it takes your mailThe Microsoft 2025 bar: SPF, DKIM, and DMARC with at least one aligned pass.- 550 5.7.509: rejected because your DMARC policy says rejectYour own p=reject, honored by Microsoft; fix the sender's alignment, not the policy.
- 550 5.7.511: Microsoft refuses mail from your sending IPA banned IP at Microsoft; find it, check for abuse, then email delist@microsoft.com.
- 550 5.7.606: your sending IP is banned at MicrosoftAny code from 5.7.606 to 5.7.649 is a banned IP; find whose it is, then use the portal.
550 5.7.28: Gmail flagged an unusual rate of unsolicited mailA rate verdict on the sending IP, not the message; stop the burst and rebuild slowly.- 421 4.7.0: Gmail is temporarily deferring your mailOne code covers eight reasons; the words after it say which one to fix.
421 4.7.0 [TS01, TS02, TS03]: Yahoo is deferring mail from your IPYahoo's TS deferrals are a reputation throttle; pace, cut complaints, then ask support.- 5.1.1 user unknown: the mailbox does not existTypos, departed staff, stale lists, and why to suppress after the first bounce.
- 5.1.2: the recipient's domain does not exist in DNSHow to tell a typo from a dead domain with one DNS lookup.
- 5.2.1: the mailbox exists but is disabledThe mailbox exists but is switched off; suppress it and restore on a positive signal.
- 5.2.2 mailbox full: the recipient is over quotaOver quota is their problem to fix, but weeks of it is a list-hygiene signal.
- 5.2.3: your message is too large for the recipient's mailboxOne mailbox's size limit, the encoding overhead, and the ways around a large file.
- 5.3.4: your message exceeds the receiving system's size limitThe system-wide size ceiling, why encoded mail is bigger, and the link fix.
- 4.4.1: connection to the recipient's mail server timed outWhy a timeout is almost always the receiving side, and the two cases where it is yours.
- 4.4.7: the message expired before it could arriveThe retry window ran out; the earlier error quoted in the bounce is the real cause.
- 5.4.4: no route to the recipient's domainDNS answered but no mail route exists; check their MX records before you act.
- 5.7.10: read the SMTP error beside the codeGmail's 523 5.7.10 is a pipelining error; read the text before touching TLS.
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