If you have postfix installed with OpenSSL support compiled in, you can enable SSL connections by editing two configuration files. First, add the following to /etc/postfix/main.cf:

smtpd_use_tls = yes
#smtpd_tls_auth_only = yes
smtpd_tls_key_file = /etc/postfix/newkey.pem
smtpd_tls_cert_file = /etc/postfix/newcert.pem
smtpd_tls_CAfile = /etc/postfix/cacert.pem
smtpd_tls_loglevel = 3
smtpd_tls_received_header = yes
smtpd_tls_session_cache_timeout = 3600s
tls_random_source = dev:/dev/urandom

Then, simply uncomment this line in /etc/postfix/master.cf:

smtps     inet  n       -       n       -       -       smtpd

Make sure to keep tabs between the elements in the master.cf file.