Exchange Online Protection
Exchange Online Protection (EOP) is the spam filter for Microsoft 365, the business productivity suite from Microsoft. It provides businesses with spam filtering, block lists, and firewall protection. If a message is going to spam or getting blocked, there are typically instructions for the email admin to fix the specific error in Exchange documentation.
Codes
- 421↑
- 421 4.3.2 The maximum number of concurrent connections per resource forest has exceeded a limit, closing transmission channel. example.com. PRX8 [example.com]
- 451↑
- 451 4.7.0 Temporary server error. Please try again later. PRX4 NextHop: example.com [example.com]
- 451 4.7.500 Server busy. Please try again later from [x.xx.xx.x]. (AS77712344) [#.prod.protection.outlook.com]
- 550↑
550 5.4.1 [email@example.com]: Recipient address rejected: Access denied [example.com]
Usually this bounce means the recipient address is invalid, but sometimes it can be caused by internal setup or routing issues. Reach out to the recipient's mail admin to verify.
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)
Exchange Online checks the sending IP against the public block list Spamhaus. Spamhaus is a very reputable block list with few false-positive listings. You must cooperate with Spamhaus volunteers to resolve the listing. Remember that even once the listing is removed, it may take receivers up to 24 hours to refresh their cache and to see delivery resume.
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]
First, the forward DNS lookup (domain name to IP address) of your sending IP should match the HELO value set in your mailserver. Then Exchange Online checks this HELO domain against the public block list Spamhaus. Spamhaus is a very reputable block list with few false-positive listings. You must cooperate with Spamhaus volunteers to resolve the listing. Remember that even once the listing is removed, it may take receivers up to 24 hours to refresh their cache and to see delivery resume.
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)