We've just completed a deployment and migration from Exchange 2010 to Exchange 2016.
Our ticket system connects to an Exchange mailbox to generate tickets. Connections from the ticket system to Exchange are failing with a timeout error.
The server running the ticket system is CentOS 7. I SSH'd into it, opened telnet and was able to connect to our Exchange 2016 servers. After connecting I get no response to "HELLO" or "EHLO" and then get disconnected due to inactivity.
Using 3 other CentOS servers, one on the same subnet as the ticket system box, I was able to successfully telnet into Exchange and get a response to my "HELLO" and "EHLO".
Why would Exchange 2016 be ignoring a single server trying to connect to it?
I checked the SMTP logs and found nothing in there but the connection and eventual timeout.
I checked the IP block list in Exchange and it's blank.
There are no firewalls/IDS/IPS involved between Exchange and the ticket system.