Services are restored on mail and shell servers. There wasn’t really a problem with the shell server, rather it was an issue with the server that held the mail spool files.
For some reason, after an upgrade the network interfaces all went away. After a reboot it mysteriously revived. It would appear there are bugs in the systemd startup files in 16.04 yet.
While everything is currently backup, I am going to upgrade the operating system on the machine that holds users home directories so there will be another short interruption but once it is complete all operating system software will be current.