5.7.26: Official Definition
5.X.X
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that start with 5.X.X:
A permanent failure is one which is not likely to be resolved by resending the message in the current form. Some change to the message or the destination must be made for successful delivery.
X.7.26
[RFC3463] (Standards Track) from the The Internet Engineering Task Force (IETF) definition for codes that end with X.7.26:
This is not an official RFC code, however it is a common error reported by Google, including these variants:
Unauthenticated email from domain-name is not accepted due to domain's DMARC policy. Please contact the administrator of domain-name domain. If this was a legitimate mail please visit Control unauthenticated mail from your domain to learn about the DMARC initiative. If the messages are valid and aren't spam, contact the administrator of the receiving mail server to determine why your outgoing messages don't pass authentication checks.
This message does not have authentication information or fails to pass authentication checks (SPF or DKIM). To best protect our users from spam, the message has been blocked. Please visit Prevent mail to Gmail users from being blocked or sent to spam for more information.
This message fails to pass SPF checks for an SPF record with a hard fail policy (-all). To best protect our users from spam and phishing, the message has been blocked. Please visit Prevent mail to Gmail users from being blocked or sent to spam for more information.
