SMTP

There are a variety of SMTP services that you can configure on your mail server. If you plan on receiving email from the internet, you'll definitely need a Port 25 service.
For your internal users, you'll likely also want SSL- and TLS-encrypted SMTP services that require them to authenticate before they can send mail. Those two types of services can also be used externally, allowing users to securely authenticate to your server and relay mail through it. Since the users are required to authenticate in order to send mail, you can disable some of your spam-prevention measures are those SMTP services

For some excellent information on how these SMTP services are configured, see John Simpson's Setting up an SMTP Service and Issues with SSL/TLS and AUTH pages.
tcpserver Tweaks
tcpserver rules can be adjusted to change settings in your SMTP services depending upon the address from which a client is connecting. You can, for example, disable greylisting or RBL checks for certain IP address ranges.