Isso é com o SASL. Key é a chave, pem é o arquivo CA e crt e certificado emitido.
/etc/dovecot/conf.d/main.cf:
readme_directory = /usr/share/doc/postfix-2.6.6/README_FILES
virtual_alias_maps = hash:/etc/postfix/virtual
sender_bcc_maps = hash:/etc/postfix/bcc
mailbox_command = /usr/bin/procmail-wrapper -o -a $DOMAIN -d $LOGNAME
home_mailbox = Maildir/
smtpd_sasl_auth_enable = yes
broken_sasl_auth_clients = yes
smtpd_recipient_restrictions = permit_mynetworks permit_sasl_authenticated reject_unauth_destination
mailbox_size_limit = 0
allow_percent_hack = no
## Specify the keys/certificates
smtpd_tls_key_file = /etc/pki/tls/private/localhost.key
smtpd_tls_CAfile = /etc/pki/tls/cert.pem
smtpd_tls_cert_file = /etc/ssl/certs/localhost.crt
smtpd_tls_security_level = may
##
/etc/dovecot/master.cf:
# ==========================================================================
# service type private unpriv chroot wakeup maxproc command + args
# (yes) (yes) (yes) (never) (100)
# ==========================================================================
smtp inet n - n - - smtpd -o smtpd_sasl_auth_enable=yes
smtps inet n - n - - smtpd -o smtpd_sasl_auth_enable=yes