Ubuntu 19.04
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Mail Server : SSL/TLS Setting2019/04/26

Configure SSL/TLS to encrypt connections.
SMTP-Submission uses 587/TCP(used STARTTLS), SMTPS uses 465/TCP, POP3S uses 995/TCP, IMAPS uses 993/TCP.
[2] Configure Postfix and Dovecot.
vi /etc/postfix/main.cf
# add to the end

smtpd_use_tls = yes
smtp_tls_mandatory_protocols = !SSLv2, !SSLv3
smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3
smtpd_tls_cert_file = /etc/letsencrypt/live/mail.srv.world/fullchain.pem
smtpd_tls_key_file = /etc/letsencrypt/live/mail.srv.world/privkey.pem
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache

vi /etc/postfix/master.cf
# line 17-21: uncomment like follows

submission inet n       -       y       -       -       smtpd
  -o syslog_name=postfix/submission
#  -o smtpd_tls_security_level=encrypt
  -o smtpd_sasl_auth_enable=yes
  -o smtpd_tls_auth_only=yes

# line 29-31: uncomment like follows

smtps     inet  n       -       y       -       -       smtpd
  -o syslog_name=postfix/smtps
  -o smtpd_tls_wrappermode=yes

vi /etc/dovecot/conf.d/10-ssl.conf
# line 6: change

ssl =
# line 12,13: uncomment and specify certificates

ssl_cert = <

ssl_key = <
systemctl restart postfix dovecot

[3] For Client's settings, ( Mozilla Thunderbird )
Open account's property and move to [Server Settings] on the left pane, then Select [STARTTLS] or [SSL/TLS] on [Connection security] field on the right pane. (this example shows to select [STARTTLS])
[4] Move to [Outgoing Server] on the left pane, then Select [STARTTLS] or [SSL/TLS] on [Connection security] field. Furthermore, change port to the used port. ([STARTTLS] uses [587], [SSL/TLS] uses 465, this example shows to select [STARTTLS])
[5] Make sure possible to send or receive Emails normally.
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