Enhanced SMTP status code · RFC 3463 · IANA
5.7.26 Multiple authentication checks failed
The message failed more than one authentication check (for example SPF and DKIM, and so DMARC), without saying which (RFC 7372).
- Class: Permanent failure
Class 5: a permanent failure. Resending the same message will not help: something about the message, the sender or the destination must change (RFC 3463 §3.1).
Also seen as:
4.7.26 - Subject: X.7 · Security and policy
Policies (per-host or per-recipient filters) and cryptographic operations; up to the sender, the recipient or both.
- Associated basic codes (IANA registry)
How 5.7.26 appears at providers
The exact text from the provider's documentation, as you see it in the bounce, and what it means.
Gmail
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550 5.7.26
This email has been blocked because the sender is unauthenticated. Gmail requires all senders to authenticate with either SPF or DKIM. Authentication results: DKIM = did not pass SPF [domain-name] with ip: [ip-address] = did not pass.
The message passes neither SPF nor DKIM, and Gmail requires at least one of them from every sender. Check with Mail delivery test.
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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 with the ip: [ip-address]. To best protect our users from spam and phishing, the message has been blocked.
The MAIL FROM domain has an SPF record ending in
-all, and the sending IP address is not authorised in it. Add the server to SPF or send through an authorised one. Check with SPF generator. -
550 5.7.26
Unauthenticated email from domain-name is not accepted due to domain's DMARC policy. Contact the administrator of domain-name domain if this was legitimate email.
The message fails DMARC, and the From domain's policy asks for rejection. Legitimate mail must pass SPF or DKIM aligned with From. Check with Mail delivery test.
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451 4.7.26
Unauthenticated email from domain-name is not accepted due to domain's DMARC policy, but temporary DNS failures prevent authentication. Please contact the administrator of domain-name domain if this was a legitimate email.
The From domain has a DMARC policy that rejects unauthenticated mail, but temporary DNS failures prevented the check. Make sure the domain's DNS servers answer. Check with Email DNS check.
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421 4.7.26
This email has been rate limited because it is unauthenticated. Gmail requires all senders to authenticate with either SPF or DKIM. Authentication results: DKIM = did not pass SPF domain-name with ip: ip-address = did not pass.
The message passes neither SPF nor DKIM, and Gmail requires at least one of them from every sender. Check with Mail delivery test.
Microsoft 365 / Exchange Online
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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
A message sent over IPv6 must pass SPF or DKIM, and this one is not signed. Check with Mail delivery test.
What you can check here
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026