Every email server that publicly receives emails from the Internet must have a valid MX record in the DNS. This rule checks if the email came from a server that also accepts emails for sender's domain. IMPORTANT: Do not enable this rule without reading the following section.
Although use of this rule will eliminate almost all spam messages, there is a high probability that you will lose good emails as well. However, if configured correctly it can reduce spam while preserving good messages. Consider the following scenario where this rule is useful.
Spammer's often forge the sender's domain name, for example:
Received: from ([126.96.36.199])In the above example it seems that you are sending yourself an email, whereas the email actually originated from 188.8.131.52 that happens to be a server on the Internet.
Subject: Save money...
For cheap software visit http://www.spammer.com
If you know which email server(s) are used to send out-bound emails from your company you can enable this rule, which will make sure that any email where the sender's domain matches your company's domain originates from one of the known IP address.
# Line starting with a # sign is a comment
# Put one domain per line.
Additionally, you can put a source IP address in the Whitelist to accept any message from certain sources. If an IP address is white listed, this rule will be ignored.