suddenly 'User unknown'?

[Dan White]

Nov 27 15:18:35 dell2600-1 sendmail[4798]: wARFIZXZ004798: from=brad,
size=44, class=0, nrcpts=1,
Nov 27 15:18:36 dell2600-1 sendmail[4801]: AUTH=client, relay=localhost, mech=, bits=0
Nov 27 15:18:36 dell2600-1 sendmail[4801]: wARFIZvh004799: wARFIavg004801: DSN: User unknown

What do your cyrus syslog entries say?

localhost.localdomain> ver
name : Cyrus IMAPD
version : v2.4.17-Invoca-RPM-2.4.17-7.el6 d1df8aff 2012-12-01
duplicatedeliver: false
lastpop: 24-Oct-2013 21:04:43 +0100
lastupdate: 27-Nov-2018 04:00:00 +0000
partition: default
pop3newuidl: true
sharedseen: false
size: 8489796

Does the output of 'lm' look correct?

and /etc/imapd.conf
configdirectory: /var/lib/imap
partition-default: /var/spool/imap
admins: cyrus
sievedir: /var/lib/imap/sieve
sendmail: /usr/sbin/sendmail
hashimapspool: true
sasl_pwcheck_method: auxprop
#
sasl_auxprop_plugin:sql
#
allowplaintext: no
unixhierarchysep: yes
virtdomains: userid
#