This is a simple guide to building an email gateway which will perform anti-spam and anti-virus filtering prior to delievering email to its intended destination.

I also make use of a sendmail milter to verify the recpients of email messages are valid, and drop the messages if the recpient is found to be invalid. Recipient filtering not only reduces the amount of work that the gateway has to do (as it doesn't perform any anti-spam analyasis etc on the message) but reduces the load on the destination server(s) and protects them.

The milter works by simply connecting to the destination server and testing if it accepts the recipient address prior to accepting the rest of the email body from the machine which is connected to it. If you use Exchange as your destination server, make sure it doesn't accept invalid recipients and returns an NDR which is the default. If the recipient isn't valid, the miltor returns to the SMTP server which is attempting to send email to it that the mail box is full or invalid.