Email provider · 5.x.x permanent failure
Every SMTP 550 response on record from Outlook (hotmail.com, outlook.com, msn.com, live.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 5.5.0 Requested action not taken: mailbox unavailable. [#.eop-nam12.prod.protection.outlook.com]
The address does not exist, or is no longer active, at this provider.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.
smtp;550 5.7.1 Service unavailable, MailFrom domain is listed in Spamhaus. To request removal from this list see https://www.spamhaus.org/query/lookup/ (S8002) [#.eop-nam02.prod.protection.outlook.com]
The sending IP or domain is on a blocklist the receiver checks, so mail stays refused until the listing clears. Find which list flagged you, fix the underlying cause, then request removal. Delivery can stay blocked for up to a day after delisting while caches refresh.
Provider documentation
smtp;550 5.7.1 Unfortunately, messages from [x.xx.xx.xx] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3150). You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors. [#.eop-nam02.prod.protection.outlook.com]
The sending IP or domain is on a blocklist the receiver checks, so mail stays refused until the listing clears. Find which list flagged you, fix the underlying cause, then request removal. Delivery can stay blocked for up to a day after delisting while caches refresh.
Provider documentation
smtp;550 5.4.1 All recipient addresses rejected : Access denied [#.eop-nam02.prod.protection.outlook.com]
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.
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 Outlook, 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 Outlook's postmaster documentation.
