Replace SSL certificate

The SSL certificate is located at /etc/ssl/mail/mail.{key,crt}.

You can replace it by just copying over your own files.

For proper SSL operation the /etc/ssl/mail/mail.crt file should contain the cert and the intermediate/bundle certificates provided by your certificate authority.

Services effected and necessary to restart are postfix, dovecot and nginx/apache2. If you're running Debian or Ubuntu, you can simply run (assuming you're using Nginx):

sudo service postfix restart
sudo service dovecot restart
sudo service nginx restart

You can use http://www.checktls.com/ to check proper operation of Postfix as a receiving MTA.