Hello Hakabe!
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 hereopenssl 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 filechmod 640 /etc/nginx/ssl/newmail.key
chown root:ssl-cert /etc/nginx/ssl/newmail.key
You have to enable the dovecot SSL option here/etc/dovecot/conf.d/10-ssl-conf
And let postfix know about your SSL certificate here/etc/postfix/main.cf
Once you got everything set up, restart/reload the servicesservice 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.
D.