Add/change settings in config_local.php
 # vi /etc/squirrelmail/config_local.php

 $smtpServerAddress  = 'localhost';
 $smtpPort           = 465;
 $smtp_auth_mech     = 'login';
 $use_smtp_tls = true;

 Add settings in SMTPS run script
 # vi /var/qmail/supervise/smtps/run

 export SMTPS=1
 export FORCETLS=0
 export SMTPAUTH="!"

 Restart qmail
 # qmailctl stop
 # qmailctl start