I have an SMTP relay server that is setup and working correctly on a 2008 server.
A new 2016 server was created at the same site and SMTP was setup using the exact same settings as the 2008 server but it is not working.
Delivery is done using xxxxxx.mail.protection.outlook.com
The new server has a different internal IP, external IP and server name.
There is no on-premise Exchange server and there are no connectors setup in Office 365 Exchange admin center that would be blocking the new server by IP.
I've also tested using a third party SMTP mail sender on both servers (to ensure no IIS problems) and it only works on the old server.
What else could be causing the new server to be rejected by Office 365?
Thank you for any help.