I will be performing multiple reboots of the client mail server tonight after 11pm to troubleshoot the issue that causes it to hang during a systemd initiated reboot. This is a problem with the new Hirsute release of Ubuntu but I managed to fix my workstation by removing a bunch of unnecessary cruft and hope that I can do the same with the mail server. Because I did quite a lot at once on my workstation, I don’t know exactly what it was that it was getting stuck on. This may also cause a delay if you attempt to login to a shell server while a reboot is in progress but if this happens just wait a couple of minutes and try again. The down times will be relatively brief (2-3 minutes).