Spam filter / gateway
Exchange Online Protection SMTP Error Codes and Rejection Messages
Exchange Online Protection is the filtering layer in front of every Microsoft 365 mailbox, providing spam filtering, blocklists and connection filtering. Specific errors usually map to a documented fix in Microsoft's guidance.
- Codes on record
- 3
- Responses
- 7
SMTP 4211 response
A temporary problem. The sending server keeps retrying for a while, and delivery often recovers on its own once the underlying issue clears. See SMTP 421 across every provider
SMTP 4512 responses
A temporary problem. The sending server keeps retrying for a while, and delivery often recovers on its own once the underlying issue clears. See SMTP 451 across every provider
SMTP 5504 responses
smtp;550 5.4.1 [[email protected]]: Recipient address rejected: Access denied [example.com]
smtp;550 5.7.1 Service unavailable, Client host [x.xx.xx.xx] blocked using Spamhaus. To request removal from this list see https://www.spamhaus.org/query/ip/0.0.0.0 (AS16012611)
550 5.7.1 Service unavailable, Helo domain is listed in Spamhaus. To request removal from this list see https://www.spamhaus.org/query/lookup/ (S8001) [#.prod.protection.outlook.com]
smtp;550 5.7.606 Access denied, banned sending IP [x.xx.xx.xx]. To request removal from this list please visit https://sender.office.com/ and follow the directions. For more information please go to http://go.microsoft.com/fwlink/?LinkID=526655 (AS16012609)
A hard failure. The receiving server has refused the message for good and will not accept an unchanged retry without a change on the sending side. See SMTP 550 across every provider
Exchange Online Protection bounce questions
›Why is Exchange Online Protection bouncing my email?
Exchange Online Protection is the filtering layer in front of every Microsoft 365 mailbox, providing spam filtering, blocklists and connection filtering. Specific errors usually map to a documented fix in Microsoft's guidance. Match the exact wording of your bounce to one of the 7 responses on this page to find the specific reason and fix.
›How do I get past the Exchange Online Protection spam filter?
Authenticate with SPF, DKIM and DMARC, keep your sending IP and domain reputation clean, mail only engaged recipients, and honour unsubscribes. Exchange Online Protection also publishes postmaster guidance, linked on this page.
