There were problems with the pop/imap/smtp service this morning after the reboots. At some point between this reboot and the previous, a Ubuntu update overwrote my systemd configuration file for dovecot, the imap/pop3 server. My changes to this file were designed to cause dovecot startup to wait until after the /misc file system, where I have the encryption certificates for the domain, mounted.
Because those were gone, the encryption certificates were unavailable so dovecot failed to start. Because dovecot SASL is used for authentication in postfix, postfix also failed to function on the client mail server. This was corrected around 7:30AM Pacific time.