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.
7 responses across 3 reply codes
SMTP 421
1 responses
SMTP 451
2 responses
SMTP 550
4 responses
5.4.1
Exchange Online denied access for this recipient. Often the address is invalid. Confirm with their mail admin if you believe it should work.
5.7.1
Exchange Online blocked your sending IP on Spamhaus. Delist with Spamhaus, then wait for caches to refresh before retrying.
5.7.1-2
Exchange Online blocked the HELO domain on Spamhaus. Match HELO to your forward DNS, delist if needed, then retry after caches refresh.
5.7.606
Exchange Online banned this sending IP. Submit a removal request at sender.office.com, then pause volume until the ban lifts.
Response catalog adapted from the MIT-licensed SMTP Field Manual by ActiveCampaign / Postmark.