TLS configuration for postfix

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1. mkdir -p /etc/postfix/ssl
2. Generate self-signed certificate using:

   openssl req -new -x509 -days 999 -nodes -out postfix.pem -keyout postfix.pem

3. chown postfix:postfix postfix.pem
4. chmod 400 postfix.pem
5. Add following lines to /etc/postfix/main.cf

   smtpd_tls_cert_file = /etc/postfix/ssl/postfix.pem
   smtpd_tls_key_file = $smtpd_tls_cert_file
   smtpd_tls_security_level = may
   smtpd_tls_received_header = yes
   smtp_tls_security_level = may
   smtp_tls_note_starttls_offer = yes

   where lines starting with smtp_ configure postfix to also send outgoing emails using TLS.

6. Add following lines after commented smtps line in /etc/postfix/master.cf

   smtps   inet n   -   n   - - smtpd
   -o smtpd_sasl_auth_enable=yes
   -o smtpd_reject_unlisted_sender=yes
   -o smtpd_recipient_restrictions=permit_sasl_authenticated,reject
   -o broken_sasl_auth_clients=yes

7. service postfix restart
8. Verify using "netstat -alnp | grep master" that postfix is listening on both port 25 and port 465

Note that if smtp auth is enabled, then disabling plaintext auth over nonencrypted channels using:

      smtpd_sasl_security_options = noanonymous, noplaintext
      smtpd_sasl_tls_security_options = noanonymous

is causing postfix to not work. Hence we depend on user to prefer TLS over unecrypted channel for plaintext authentication.

Please also refer to CentOS 7.x Enable submission 587 port for postfix

Steps learned from http://www.postfix.org/TLS_README.html

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