Today I logged in to the physical machine that the mail virtual host runs on and it died.
I was able to determine that the cause was an out of memory condition on the machine even though it normally has about 18gb free RAM available.
I believe it died when I logged in because it was probably a buggy version of x2goserver that caused it. The x2goserver had been acting weird on that machine, giving me a tiny display instead of a normal full screen display.
I use a nightly builds version because the stable release is incompatible with the rest of the graphical environment on that machine. That machine requires xrandr version 1.3 or greater, the stable x2goserver uses version 1.2, but the version in nightly builds uses 1.3.
The nightly builds may have serious bugs since it is the development version and in this case it appeared it had a bug that ate up all memory. I have since de-installed and re-installed the x2goserver and all associated libraries and applications. Now it works properly and it doesn’t crash mail.
A stable release with this version of xrandr is expected around the second week of February but in the meantime I will not re-install unless there is some other problem.
I also wrote a small script that runs once a minute and checks to make sure mail is alive and if not boots it so that if this does occur again it will automatically come back up. It logs it when this occurs with a time stamp so I will be able to examine logs on the machine to see what else was going on at the time it went down.