Are you on the same LAN as the SME? You can only access the SMTP server to send mail if you're on the same LAN. Otherwise anyone anywhere would be able to send mail from your server. If you want to add authentication to the SMTP server for external access, use Damien Curtain's SASL contrib.
And yes, if your SME has a private IP behind a router with the public IP, then you'll need to forward any ports to it for services that you wish to use.