Mail errors
What does my mail error mean?
Find the words from your bounce or your message headers below.
Last read from the sources at the foot of this page.
Gmail bounce
Mail Delivery Subsystem
550 5.7.26 Unauthenticated email from domain-name is not accepted due to domain's DMARC policy
Find yours below: each page quotes its source.
01
Which Gmail errors are here?
Bounces from Gmail and Google Workspace about authentication, reputation and message format.
- 550 5.7.26 Unauthenticated email from domain-name is not accepted due to domain's DMARC policy
- 451 4.7.26 Unauthenticated email from domain-name is not accepted due to domain's DMARC policy, but temporary DNS failures prevent authentication
- 550 5.7.26 This email has been blocked because the sender is unauthenticated
- 550 5.7.26 The (E)MAIL FROM domain [domain-name] has an SPF record with a hard fail policy (-all) but it fails to pass SPF checks
- 421 4.7.40 Your email has been rate limited because the sending domain doesn't have a DMARC record
- 421 4.7.32 Your email has been rate limited because the From: header (RFC5322) in this message isn't aligned with either the authenticated SPF or DKIM organizational domain
- 421 4.7.27 Your email has been rate limited because SPF authentication didn't pass for this message
- 421 4.7.30 Your email has been rate limited because DKIM authentication didn't pass for this message
- 451 4.7.24 The SPF record of the sending domain has one or more suspicious entries
- 550 5.7.25 This message was blocked because the sending IP address doesn't have a PTR record
- 421 4.7.29 Your email has been rate limited because you're not using a TLS connection
- 421 4.7.28 Gmail has detected an unusual rate of email
- 550 5.7.1 This message is likely suspicious due to the very low reputation of the sending domain
- 550 5.7.1 This message is likely suspicious due to the very low reputation of the sending IP address
- 550 5.7.1 This message is likely unsolicited email
- 421 4.7.0 This message is suspicious due to the nature of the content or the links within
- 550 5.7.1 The IP you're using to send email is not authorized to send email directly to our servers
- 550 5.7.1 Messages missing a valid Message-ID: header are not accepted
- 550 5.7.1 Messages missing a valid address in the From: header, or having no From: header, are not accepted
- 550 5.7.1 This message does not meet IPv6 sending guidelines regarding PTR records and authentication
02
Which Microsoft 365 and Outlook errors are here?
Bounces from Microsoft 365 and Outlook.com, and the compauth verdict in a message's headers.
- 550; 5.7.515 Access denied, sending domain [SendingDomain] does not meet the required authentication level.
- 5.7.509 Access denied, sending domain [$SenderDomain] does not pass DMARC verification and has a DMARC policy of reject.
- 5.7.23 The message was rejected because of Sender Policy Framework violation
- 4.7.26 Access denied, a message sent over IPv6 [2a01:111:f200:2004::240] must pass either SPF or DKIM validation, this message is not signed
- 5.7.25 Access denied, the sending IPv6 address [2a01:111:f200:2004::240] must have a reverse DNS record
- 5.7.750 Service unavailable. Client blocked from sending from unregistered domains
- 5.7.367 Remote server returned not permitted to relay
- 5.7.512 Access denied, message must be RFC 5322 section 3.6.2 compliant
- compauth=fail reason=000
- compauth=fail reason=001
- compauth=fail reason=010
- dmarc=bestguesspass
03
Which SPF, DKIM and DMARC results are here?
Results in a message's headers and in DMARC reports, as the standards define them.
Sources
Where does this come from?
- Google Workspace: Gmail SMTP errors and codesknowledge.workspace.google.comRead 14 October 2026
- Gmail: Email sender guidelinessupport.google.comRead 14 October 2026
- RFC 7208, Sender Policy Frameworkrfc-editor.orgRead 14 October 2026
- Microsoft: Outlook's new requirements for high-volume senderstechcommunity.microsoft.comRead 14 October 2026
- Microsoft: Email non-delivery reports and SMTP errors in Exchange Onlinelearn.microsoft.comRead 14 October 2026
- Microsoft: Email message headers in Microsoft 365learn.microsoft.comRead 14 October 2026
- RFC 6376, DomainKeys Identified Mail (DKIM) Signaturesrfc-editor.orgRead 14 October 2026
- Zoho Mail: DKIM configurationzoho.comRead 14 October 2026
- Google Workspace: About DMARC reportsknowledge.workspace.google.comRead 14 October 2026
Related
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