Gmail delivery troubleshooting
Gmail SMTP errors, decoded by evidence
Diagnose Gmail SMTP rejections involving SPF, DKIM, DMARC, PTR, TLS, routing and message content.
Choose the exact response from Gmail. Provider wording matters because the same enhanced status code can mean different things elsewhere.
Gmail 550 5.7.24Understand Gmail 550 5.7.24, check a suspicious SPF record for syntax, duplicate policies and broken dependencies, and repair it without removing valid senders.
Gmail 550 5.7.30Diagnose Gmail 550 5.7.30 DKIM authentication failures with the exact selector and signing domain from message headers before changing DNS.
Gmail 552 5.7.0Fix Gmail 552 5.7.0 when message or attachment content presents a security issue and is blocked.
Fix Gmail 421 4.7.26 unauthenticated email rate limitingFix Gmail 421 4.7.26 by separating a temporary unauthenticated-mail rate limit from current SPF, DKIM, and DMARC configuration problems.
Fix Gmail 421 4.7.27 SPF rate limitingFix Gmail 421 4.7.27 by checking the bulk sender's actual IP and MAIL FROM SPF path while preserving temporary retry behavior.
Fix Gmail 421 4.7.28 unusual email rate limitingResolve Gmail 421 4.7.28 temporary rate limits caused by unusual or unsolicited mail volume, IP, domain, or URL reputation.
Fix Gmail 451 4.7.23 PTR rate limitingFix Gmail 451 4.7.23 when a sending IP lacks PTR or forward-confirmed reverse DNS and mail is temporarily rate limited.
Fix Gmail 451 4.7.24 suspicious SPF rate limitingFix Gmail 451 4.7.24 by reviewing suspicious SPF entries and dependencies while preserving temporary retry behavior.
Fix Gmail 451 4.7.26 temporary authentication failureDiagnose Gmail 451 4.7.26 when DMARC would reject unauthenticated mail but a temporary DNS failure prevents authentication.
Fix Gmail 550 5.1.1 : account does not existResolve Gmail 550 5.1.1 by verifying the recipient address, account status, aliases, and recent mailbox changes.
Fix Gmail 550 5.7.25 PTR and reverse DNS failureFix Gmail 550 5.7.25 when a sending IP has no PTR record or forward-confirmed reverse DNS does not match.
Fix Gmail 550 5.7.28 unsolicited email blockingResolve Gmail 550 5.7.28 when unusual unsolicited email from a sending IP is permanently blocked for the attempt.
Fix Gmail 550 5.7.29 : email was not sent over TLSFix Gmail 550 5.7.29 when bulk email reaches Gmail without a negotiated TLS connection.
Fix Gmail 553 5.1.2 recipient domain not foundFix Gmail 553 5.1.2 by checking the recipient domain for spelling, trailing punctuation, registration, delegation, and mail DNS.
Fix Gmail 554 5.6.0 : email message is malformedFix Gmail 554 5.6.0 by validating RFC message structure, headers, MIME boundaries, and the software generating email.
Fix Gmail 550 5.7.26Diagnose Gmail 550 5.7.26 safely. Separate unauthenticated-mail, SPF hard-fail and DMARC-policy branches before changing email DNS.
Fix Gmail 550 5.7.27 SPF authentication failedDiagnose Gmail 550 5.7.27 SPF authentication failures with the actual sending IP and MAIL FROM domain. Find the safe next DNS or sender configuration step.
Fix Gmail 550 5.7.40 DMARC policy requiredDiagnose Gmail 550 5.7.40 with a current RFC 9989 DMARC policy check. Distinguish a missing or invalid policy from temporary DNS failures and message alignment.
Primary sources: Gmail SMTP errors.