Symptoms
An addon domain or subdomain's SSL certificate is not returned by the Dovecot service when making an IMAP or POP3 connection.
Description
Dovecot uses the /etc/dovecot/sni.conf for the SSL configuration. The domain will not be added to the configuration if there are no email accounts.
Workaround
Create an email address to the addon domain or subdomain:
How can I create an email account?
After, rebuild the Dovecot SNI configuration:
/scripts/build_mail_sni --rebuild_dovecot_sni_conf
/scripts/restartsrv_dovecot