CentOS 7
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SSL Settings
Configure SSL to encrypt connections.
[2] Configure Postfix and Dovecot for SSL.
[[email protected] ~]#
vi /etc/postfix/main.cf
# add to the end

smtpd_use_tls = yes
smtpd_tls_cert_file = /etc/pki/tls/certs/server.crt
smtpd_tls_key_file = /etc/pki/tls/certs/server.key
smtpd_tls_session_cache_database = btree:/etc/postfix/smtpd_scache
[[email protected] ~]#
vi /etc/postfix/master.cf
# line 26-28: uncomment

smtps       inet   n       -       n       -       -       smtpd
  -o syslog_name=postfix/smtps
  -o smtpd_tls_wrappermode=yes
[[email protected] ~]#
vi /etc/dovecot/conf.d/10-ssl.conf
# line 8: change

ssl =
# line 14,15: specify certificates

ssl_cert = <

ssl_key = <
[[email protected] ~]#
systemctl restart postfix dovecot

[3] If Firewalld is running, allow SMTPS/POP3S/IMAPS services. SMTPS uses 465/TCP, POP3S uses 995/TCP, IMAPS uses 993/TCP.
[[email protected] ~]#
firewall-cmd --add-service={pop3s,imaps} --permanent

[[email protected] ~]#
firewall-cmd --add-port=465/tcp --permanent

[[email protected] ~]#
firewall-cmd --reload

[4] For Client's settings, ( Mozilla Thunderbird )
Open account's property and move to [Server Settings] on the left pane, then Select [SSL/TLS] on [Connection security] field on the right pane.
[5] Move to [Outgoing Server] on the left pane, then Select [SSL/TLS] on [Connection security] field. Furthermore, change port to [465] like follows.
[6] If your own self-signed certificates, the warning like follows are displayed, but it's no ploblem to proceed. Then it's possible to send or receive emails through SSL/TLS connection.