Following the tutorial in “The Perfect Setup – Debian Sarge (3.1)”
http://www.howtoforge.com/perfect_setup_debian_sarge

To add TLS support in Postfix, simply add the following lines to /etc/postfix/master.cf
smtps inet n - - - - smtpd -o smtpd_tls_wrappermode=yes -o smtpd_sasl_auth_enable=yes