Symptoms
Secure connection to a mailbox fails over POP3S/IMAPS, with an error in /var/log/maillog
dovecot: pop3-login: Disconnected (no auth attempts in 0 secs): user=<>, rip=XXX.XXX.XXX.XXX, lip=XXX.XXX.XXX.XXX, TLS: SSL_read failed: error:14094412:SSL routines:ssl3_read_bytes:sslv3 alert bad certificate: SSL alert number 42, session=<kdjKIaiqCyC/t2TL>
Cause
The SSL/TLS certificate used to secure the mail server at Tools & Settings > SSL/TLS Certificates is different from the domain of the mailbox being configured.
Resolution
Secure mail on the domain by using the instructions in: How to assign a mail SSL/TLS certificate per domain (SNI support).