SMTP enhanced status code 5.7.26

What the registry says

The basic codes it travels with

• 550 Requested action not taken: mailbox unavailable (e.g., mailbox not found, no access, or command rejected for policy reasons)

In practice

550 5.7.26 is the code most senders meet after Gmail and Yahoo tightened their sender rules in 2024. Gmail uses it when mail fails both SPF and DKIM, when SPF hard-fails, and when the From domain's DMARC policy rejects the message. Microsoft uses 4.7.26 for IPv6 messages without SPF or DKIM. Aligned SPF or DKIM, plus a DMARC record, fixes it.

What the sender should do

Check SPF, DKIM and DMARC alignment for the From domain; all three should pass for bulk mail.

How providers use X.7.26

Quoted from each provider’s own error reference.

• Gmail 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. To learn about the DMARC initiative, go to Control unauthenticated mail from your domain.

• Gmail 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. To resolve this issue, go to Email sender guidelines.

• Gmail 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. For instructions on setting up authentication, go to Email sender guidelines.

• Microsoft 365 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

Common questions

What does 5.7.26 mean?

X.7.26 means the message failed more than one authentication check. Gmail uses 550 5.7.26 for unauthenticated mail and DMARC policy failures.

What should I do about 5.7.26?

Check SPF, DKIM and DMARC alignment for the From domain; all three should pass for bulk mail.

Would an email verifier have caught 5.7.26?

No. This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.

Full guides for this code

• 421 4.7.26 — rate limited, unauthenticated (Gmail)
• 550 5.7.26 — blocked, sender unauthenticated (Gmail)