We have a bit of good news. We have been able to recreate the problem on one of our Windows 2012 R2 servers. As a result, we were also able to contact Microsoft and have created a support ticket. Below is a snippet from Microsoft's tech support.

As we discussed the issue you are experiencing might happen because of the KB4338815.

This has not been identified as a bug from Microsoft yet.

But we recommend to uninstall the KB4338815 as of now. We are trying our best to get this issue resolved.

Please refer below article for more information.

 

July 10, 2018—KB4338815 (Monthly Rollup)

https://support.microsoft.com/en-us/help/4338815/windows-81-update-kb4338815

 

Thank you for your patience!

Some more info about the problem:

• Microsoft is aware of this issue and is working on it. The link above confirms a similar problem with SQL Server and IIS. We were able to do a remote screen share session with MS support and showed them the problem.
• The problem occurs when a TCP/IP server runs for a few hours and is restarted. The restart always fails because the OS thinks the port is in use. There are several reasons for the SMTP server in Xeams to restart:
  • Someone changes the configuration requiring restart
  • Entire Xeams service is restarted
  • A built-in watchdog service in Xeams restarts the SMTP server. This watchdog service attempts to restart the SMTP server if no emails are received in 10 minutes. You can configure the watchdog to wait longer before a restart occurs. This will mitigate the problem to a certain extent because the port is never released by Xeams. To do this add the following line in server.properties file.
    
    smtp.watchdog.timeout.minutes=1440
    
    This value is specified in minutes. A value of 1,400 translates into 24 hours.
• The problem is NOT restricted to port 25. It could happen on any port that is being actively used.