Bounce codes
What each SMTP bounce code means, and whose problem it is.
550 5.7.515: Microsoft wants SPF, DKIM, and DMARC before it takes your mail
Outlook and Microsoft 365 hard-reject senders that miss the 2025 bulk-sender authentication bar. What 5.7.515 means and how to pass it.
550 5.7.509: rejected because your DMARC policy says reject
Microsoft honored the p=reject your domain publishes and refused a message that failed DMARC. Why your own mail can trip it, and the alignment fix.
550 5.7.511: your sending address or domain is on Microsoft's block list
What the 5.7.511 access-denied bounce means, how domains end up banned at Outlook, and the delisting path through Microsoft sender support.
550 5.7.606: your sending IP is banned at Microsoft
The 5.7.606 to 5.7.649 range means Outlook blocked the IP itself. How to check whether it is your IP or your provider's, and how to request delisting.
550 5.7.28: Gmail flagged an unusual rate of unsolicited mail
Gmail is rejecting because the sending source looks like a spam burst. What drives 5.7.28, and how to recover the source's reputation.
421 4.7.0: Gmail is temporarily deferring mail from your IP
The unsolicited-mail deferral: Gmail is throttling the IP, not bouncing you permanently. What the 4.7.0 tempfail means and how it clears.
421 4.7.0 [TS01, TS02, TS03]: Yahoo is deferring mail from your IP
Yahoo's TS deferral codes mean reputation throttling. What each TS code implies, and how complaint rates and volume pacing clear it.
5.1.1 user unknown: the mailbox does not exist
The classic bad-address bounce. Typos, departed employees, and stale lists, plus why repeated 5.1.1 sends damage your sending reputation.
5.1.2: the recipient's domain does not exist in DNS
The receiving domain has no MX or does not resolve. How to tell a typo from a dead domain, and what to check before you retry.
5.2.1: the mailbox exists but is disabled
A suspended or deactivated recipient account. What 5.2.1 tells you, how it differs from user-unknown, and when to stop sending.
5.2.2 mailbox full: the recipient is over quota
Not your problem to fix, but worth handling well. What the over-quota bounce means and how senders should treat repeated 5.2.2 addresses.
5.2.3: your message is too large for the recipient's mailbox
The per-recipient size rejection. How 5.2.3 differs from the system-wide 5.3.4 limit, and the practical ways around a large attachment.
5.3.4: your message exceeds the receiving system's size limit
The whole receiving system, not one mailbox, refused the size. Typical limits at the big providers and how to deliver large content anyway.
4.4.1: connection to the recipient's mail server timed out
Your server could not reach theirs. Why 4.4.1 is almost always a receiving-side problem, and when a stuck queue becomes your problem.
4.4.7: the message expired before it could be delivered
Your server retried for days and gave up. What the retry-window bounce means, and how to read the earlier tempfail it usually hides.
5.4.4: no route to the recipient's domain
DNS resolved but no usable mail route exists, usually a broken MX on the receiving side. How to verify whose DNS is wrong before you retry.
5.7.10: encryption required but the connection could not use TLS
One side requires TLS and the handshake failed or was unavailable. Who enforces it, how MTA-STS changes the picture, and what to check first.
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