Email provider · 5.x.x permanent failure
Every SMTP 550 response on record from Yandex (yandex.com), 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 5.7.1 Policy rejection on the target address
The receiver rejected the message under a policy rule rather than a content or reputation score. This is usually set by the recipient's mail admin, so ask the recipient to allowlist your sending address or domain.
Provider documentation
smtp;550 5.7.1 No such user!
The address does not exist, or is no longer active, at this provider.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
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 Yandex, 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 Yandex's postmaster documentation.
