Of all the things that can happen to your SMTP server, probably the worst is having it abused as an open relay—a server that relays mail to third parties without your permission. This will consume a lot of bandwidth (which can be costly), eat up server resources (possibly slowing down or stopping other services), and can be expensive in both time and money. A more serious consequence is that your e-mail server will probably end up on one or more blacklists, and any e-mail server that refers to those lists will refuse to accept any mail from your server until you have proven it to be relay safe. If you need to use e-mail in order to carry out business, you will have a big problem.
This chapter will explain how to:
Protect Postfix from relay abuse
Differentiate between statically and dynamically assigned IP addresses
Configure relay permissions using Postfix for static IP addresses
Use Cyrus SASL for authentication from unpredictable and dynamic IP addresses...