By default, the Postfix SMTP server will accept mail only from or to the local network or domain, so that your system can't be used as a mail relay to forward bulk mail from random strangers.
The text in this document describes how you can set up more detailed anti-UCE policies that prevent delivery of unwanted email altogether, for example with sendmail-style access lists or with RBL (real-time blackhole list) name servers.
Unless indicated otherwise, all parameters described here are in the main.cf file. If you change parameters of a running Postfix system, don't forget to issue a postfix reload command.
In addition to restrictions that are specific to HELO (EHLO) command parameters, you can also specify restrictions based on the client hostname or network address.
In addition to restrictions that are specific to sender mail addresses, you can also specify restrictions based on the information passed with the HELO/EHLO command, and on the client hostname or network address.
In addition to restrictions that are specific to recipient mail addresses, you can also specify restrictions based on the sender mail address, on the information passed with the HELO/EHLO command, and on the client hostname or network address.
In addition to restrictions that are specific to ETRN domain names, you can also specify restrictions based on the information passed with the HELO/EHLO command, and on the client hostname or network address.
Note: RBL lookups are disabled by default.
A host or destination address matches $relay_domains when
its name or parent domain matches any of the names, files or lookup
tables listed in $relay_domains.