Email provider · 5.x.x permanent failure
Every SMTP 553 response on record from Google (gmail.com, googlemail.com), with the verbatim text, the enhanced status code, a severity rating and a plain-English fix.
The mailbox name is not allowed, usually because the address is malformed, the domain is wrong, or the receiver will not accept mail addressed that way.
smtp;553 5.1.2 The recipient address <[email protected]> is not a valid RFC-5321 address. - gsmtp
The receiver refused the message for good and points at an addressing problem (the recipient or sender address itself) (enhanced status 5.1.2). Use the wording above to pin down the exact fix.
Provider documentation
Postmaster & documentation
The mailbox name is not allowed, usually because the address is malformed, the domain is wrong, or the receiver will not accept mail addressed that way. At Google, it is a permanent rejection, and the specific reason is in the enhanced status code and message text of each response below.
Match your bounce text to one of the responses below and follow its guidance. Recipient-side wording means fix the address. Authentication or reputation wording (SPF, DKIM, DMARC, blocklist) means fix your sending setup, and check Google's postmaster documentation.
