Mail Encryption in Postfix
in /etc/postfix/main.cf, add:
1smtpd_tls_security_level = may
2smtp_tls_security_level = may
3smtp_tls_loglevel = 1
4smtpd_tls_loglevel = 1
Details from From http://blog.snapdragon.cc/2013/07/07/setting-postfix-to-encrypt-all-traffic-when-talking-to-other-mailservers/