Webmail, imap, and pop were unavailable from 7:15PM to approximately 8:30PM.
I restarted mail to test a new kernel before I restart the large servers tonight. For some reason dovecot automatic start failed and I did not realize it at the time.
I got a call just before 8:30, checked, and found it was not running and restarted and it started fine so no idea what went wrong earlier.
An update; I discovered what went wrong earlier is that at some point Ubuntu changed where dovecot / systemd expected the PID file to be and it required manually editing the unit file. I did that so it should be okay in future restarts.