I've just come trough a painful way to get my mail server working (http://community.rtcamp.com/t/solved-mail-server-problems/8466)
You're going to need a ssl certificate. You can create your own certificate here
openssl req -x509 -nodes -days 365 -newkey rsa:2048 -keyout /etc/nginx/ssl/newmail.key -out /etc/nginx/ssl/newmail.crt
Let's keep secure the key file
chmod 640 /etc/nginx/ssl/newmail.key
chown root:ssl-cert /etc/nginx/ssl/newmail.key
You have to enable the dovecot SSL option here
And let postfix know about your SSL certificate here
Once you got everything set up, restart/reload the services
service postfix reload
service postfix restart
service dovecot reload
service dovecot restart
And you finally get your SSL set up ready to secure your mails.
Hope it helps Regards.