The mail server is back up.  I don’t know what happened to Dovecot but it was generating errors I’ve never seen before, errors referring to “extra groups” and I was not able to gain any useful insights from Google, Bing, reboot didn’t help, debug didn’t offer any useful insights, etc.

     Finally I purged Dovecot (the imap, pop3, and mail authentication server) and re-installed from scratch.  The config files had changed significantly since when I installed it so it is possible an update changed the software to expect the new format but didn’t other to tell me.  At any rate it is back up.  There may be some rough edges yet but it is at least functioning.