550 5.7.511 is a permanent SMTP reply meaning access denied, banned sender ip (microsoft). This is a block on the sending IP, not on one message. Microsoft's reference says to email delist@microsoft.com with the full bounce code and the IP address to ask for removal.
What 550 5.7.511 means
This is a block on the sending IP, not on one message. Microsoft's reference says to email delist@microsoft.com with the full bounce code and the IP address to ask for removal.
Common causes
The IP sent spam or hit spam traps
Most commonHigh complaint rates or mail to trap addresses from that IP.
A compromised account or server used the IP
CommonSpam from a hacked mailbox or an open relay on your network.
You share the IP with a bad sender
Less commonOn shared IPs, another customer's mail can get the IP banned.
How to fix 550 5.7.511
- 1
Fix the cause before asking
Stop the spam source, secure accounts, and close any open relay.
- 2
Ask Microsoft to delist the IP
Email delist@microsoft.com with the full NDR code and the IP address, as Microsoft's reference says.
- 3
Check other lists too
Run a free blocklist check, because the same problem often lists the IP elsewhere.
Confirm your domain is set up correctly
Most delivery failures — permanent or temporary — trace back to authentication or reputation. Run a free EmailHealth check to confirm SPF, DKIM, DMARC, blacklist status, and TLS are all healthy for your sending domain. If you send in volume, free daily monitoring will alert you before a small issue becomes a flood of bounces.