Traffic Not Accepted From This IP A Microsoft 365 mail-flow note from a real tenant setup.
After setting up a new Microsoft tenant, I encountered a mail-flow problem that looked more complicated than it actually was.
Outlook itself could send and receive e-mails without problems. The issue appeared only when our self-hosted mail server tried to send through the setup using its own IP address.
The error
The error message was clear enough to be annoying, but not clear enough to solve immediately without the right context.
550 5.7.708 Service unavailable. Access denied, traffic not accepted from this IP.
Normal sending and receiving through Outlook was not affected, which made the problem easy to misinterpret at first.
The problem appeared in the mail-flow path involving our self-hosted server and its own IP address.
I tried several approaches for two days before the correct explanation became clear.
The cause
Microsoft Support explained that the new tenant had not yet built up sufficient reputation. As a result, the unknown sending IP was initially rejected.
The support message that helped
The useful part was describing the exact error instead of talking around the issue. Microsoft Support could then identify the known problem quickly.
We have set up a new tenant and unfortunately receive the following error:
550 5.7.708 Service unavailable. Access denied, traffic not accepted from this IP.
Could you please release our IP 123.345.678.910, as it may currently have a low reputation?The important detail was the exact error number 550 5.7.708. Once the issue was described precisely, Microsoft released the IP address and mail sending worked again.
What I learned
The technical lesson was simple: with Microsoft 365 mail-flow issues, the exact error code matters more than a long explanation.
Include the exact error number and the affected sending IP address.
If the tenant is newly created, say that clearly. It may be relevant to reputation handling.
A blacklisted IP can cause similar symptoms. In my case, the IP was clean.
A short, exact support request can be more useful than a long story with too many side details.
If you see 550 5.7.708 with a new Microsoft 365 tenant, do not only check your own server. The tenant and sending IP may need Microsoft-side handling.
Solved through Microsoft Support within 24 hours once the problem was described correctly.