In order for Postfix to connect to other hosts using TLS, add the following entries to your postfix main.cf file (usually /etc/postfix/main.cf). Of course, TLS has to be installed and configured for this to work.
smtp_tls_security_level = may
smtp_tls_loglevel = 1
The second line gets the TLS connection to be logged in the mail log file. Good for debugging.