Email provider · 5.x.x permanent failure
Every SMTP 550 response on record from AT&T (att.net, sbcglobal.net, swbell.net, bellsouth.net), with the verbatim text, the enhanced status code, a severity rating and a plain-English fix.
The mailbox is unavailable and the message is rejected for good. This is the catch-all permanent failure, covering everything from an unknown address to an authentication failure to a reputation block, so the wording after the number is what actually tells you the reason.
smtp;550 5.2.1 <[email protected]>... Addressee unknown, relay=[x.xx.xx.xx]
The address does not exist, or is no longer active, at this provider. Remove it from your list and check for a typo in the local part or the domain.
Provider documentation
smtp;550 5.2.1 <[email protected]>... blank mailhost - invalid address, relay=[x.xx.xx.xx]
The receiver refused the message for good and points at a mailbox problem (missing, disabled, or full) (enhanced status 5.2.1-2). Use the wording above to pin down the exact fix.
Provider documentation
smtp;550 5.7.1 Connections not accepted from servers without a valid sender domain. ## Fix reverse DNS for x.xx.xx.xx
The receiver could not match your sending IP to a valid PTR (reverse DNS) record, or your HELO name does not resolve. Set forward-confirmed reverse DNS for every sending IP.
Provider documentation
The mailbox is unavailable and the message is rejected for good. This is the catch-all permanent failure, covering everything from an unknown address to an authentication failure to a reputation block, so the wording after the number is what actually tells you the reason. At AT&T, 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 AT&T's postmaster documentation.
