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.
Search troubleshooting guides
Find a guide by SMTP code, provider, or words from the error message.
Enter at least two characters.
Gmail 550 5.7.24: The SPF record of the sending domain has one or more suspicious entriesFix Gmail 550 5.7.24 by checking the exact sending domain SPF record and correcting only confirmed syntax, dependency, or policy problems.
Gmail 550 5.7.27: This message didn't pass SPF authenticationDiagnose Gmail 550 5.7.27 using the actual sending IP and envelope MAIL FROM domain, then correct only the SPF or sender path that failed.
Gmail 421 4.7.27: SPF authentication didn't passRespond safely to Gmail 421 4.7.27: preserve queued retries, evaluate the actual sender IP and MAIL FROM SPF path, and verify recovery.
Gmail 550 5.7.29: This message was blocked because it wasn't sent over a TLS connectionFix Gmail 550 5.7.29 by restoring TLS on the final SMTP connection to Gmail.
Gmail 550 5.7.30: This message was blocked because it didn't pass DKIM authenticationDiagnose Gmail 550 5.7.30 DKIM authentication failures with the exact selector and signing domain from message headers before changing DNS.
Gmail 421 4.3.0: Temporary System Problem. Try again laterUnderstand Gmail 421 4.3.0 Temporary System Problem, retry safely, and know when to investigate the sending connection.
Gmail 421 4.7.0: Try again later, closing connectionDiagnose Gmail 421 4.7.0 by using its complete text to distinguish connection, PTR, TLS, reputation, content, and allowlist deferrals.
Gmail 550 5.2.1: The email account is inactiveUnderstand Gmail 550 5.2.1 when an account is inactive or receiving mail too quickly, and identify the recipient-side next step.
Gmail 550 5.7.0: Email relay denied or email sending deniedDiagnose Gmail 550 5.7.0 by using the complete response to separate relay settings, credentials, abuse suspension, and sending denial.
Gmail 552 5.2.2: Recipient's inbox is out of storage space and inactiveResolve Gmail 552 5.2.2 by having the recipient free Google account storage or increase its allocation before mail is sent again.
Gmail 552 5.3.4: Message exceeded Google's size limitsFix Gmail 552 5.3.4 by reducing message, attachment, or header size before sending a new message.
Gmail 553 5.1.3: Recipient address is not a valid RFC 5321 addressFix Gmail 553 5.1.3 by correcting the invalid SMTP recipient address before sending again.
Gmail 553 5.1.7: Sender address is not a valid RFC 5321 addressFix Gmail 553 5.1.7 by correcting the invalid SMTP sender address before sending again.
Gmail 450 4.2.1: Recipient receiving messages too quicklyGmail 450 4.2.1 is a temporary rate limit: retry later and use the exact bounce wording to identify whether the recipient or your Workspace relay reached a limit.
Gmail 523 5.7.10: Invalid SMTP pipelining after STARTTLSFix Gmail 523 5.7.10 by correcting an SMTP client that pipelines commands across STARTTLS instead of starting a fresh encrypted SMTP session.
Gmail 530 5.7.0: Authentication or STARTTLS requiredFix Gmail 530 5.7.0 by enabling STARTTLS when requested and authenticating the sending application with an approved Google account method.
Gmail 534 5.7.90: App password or browser login requiredResolve Gmail 534 5.7.90 by completing the requested account sign-in check or using an app password only when Google makes that option available.
Gmail 535 5.7.80: Username or password rejectedFix Gmail 535 5.7.80 by replacing rejected SMTP credentials or moving the application to an authentication method allowed by the Google account.
Gmail 550 5.7.1: Delivery not authorizedGmail 550 5.7.1 is a broad permanent rejection; match the exact response text to a policy, relay, reputation, header, rate-limit, or IPv6 problem before acting.
Gmail 552 5.7.0: "Potential security issue"Fix Gmail 552 5.7.0 by finding the attachment, archive, macro, image, or link that Gmail considers unsafe.
Gmail 451 4.7.23: The sending IP address doesn't have a PTR record, or forward DNS doesn't matchFix Gmail 451 4.7.23 when a sending IP lacks PTR or forward-confirmed reverse DNS and mail is temporarily rate limited.
Gmail 550 5.7.25: This message was blocked because the sending IP address doesn't have a PTR record, or forward DNS doesn't reference the sending IP addressFix Gmail 550 5.7.25 by making the sending IP's PTR hostname resolve forward to the same IP.
Gmail 550 5.7.28: There is an unusual rate of unsolicited email originating from your IP addressResolve Gmail 550 5.7.28 when unusual unsolicited email from a sending IP is permanently blocked for the attempt.
Gmail 451 4.7.24: The SPF record of the sending domain has one or more suspicious entriesFix Gmail 451 4.7.24 by reviewing suspicious SPF entries and dependencies while preserving temporary retry behavior.
Gmail 550 5.1.1: The email account that you tried to reach does not existResolve Gmail 550 5.1.1 by verifying the recipient address, account status, aliases, and recent mailbox changes.
Gmail 550 5.7.26: This email has been blocked because the sender is unauthenticatedDiagnose Gmail 550 5.7.26 safely. Separate unauthenticated-mail, SPF hard-fail and DMARC-policy branches before changing email DNS.
Gmail 550 5.7.40: Your message was blocked because the sending domain doesn't have a DMARC record or policyDiagnose 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.
Gmail 553 5.1.2: We weren't able to find the recipient domainFix Gmail 553 5.1.2 by checking the recipient domain for spelling, trailing punctuation, registration, delegation, and mail DNS.
Gmail 554 5.6.0: Email message is malformed. Not acceptedFix Gmail 554 5.6.0 by validating RFC message structure, headers, MIME boundaries, and the software generating email.
Gmail 421 4.7.26: This email has been rate limited because it is unauthenticatedFix Gmail 421 4.7.26 by using the reported SPF and DKIM results to authenticate the affected sending path.
Gmail 421 4.7.28: Gmail has detected an unusual rate of emailResolve Gmail 421 4.7.28 by slowing retries and investigating the IP, domain, URL, traffic, or Message-ID named in the full response.
Gmail 451 4.7.26: Unauthenticated email is not accepted due to DMARC policy, but temporary DNS failures prevent authenticationFix Gmail 451 4.7.26 by checking temporary DNS failures and restoring aligned SPF or DKIM without weakening DMARC.
Primary sources: Gmail SMTP errors.