Brian,

Here's a scenario. SpamFilter receives email from an ISP's mail server. The mail server's IP is 209.26.140.3. This resolves to "mail.netwide.net". This ISP host email for several companies/clients. One such client is logsat.com. LogSat's email is handled by that same mail server. When we at LogSat send email, our emails are in the form "user@logsat.com", but they are relayed by "mail.netwide.net". In this case, the sender's domain (logsat.com) does not match where it was sent from (mail.netwide.net), and so the rule you request would cause a legitimate email to be lost.

Since this scenario is very common, we will not be able to implement it, at least not in the way we interpreted your question. If we mis-understood you, please let us know so we may answer correctly.

Roberto Franceschetti LogSat Software