MailfilterX

MailfilterX is a Mac OS X front-end application for the UNIX based program Mailfilter. Mailfilter connects to any POP3 mail box and compares part of its content to a set of user defined filter rules.

That way the spam gets deleted directly on the mail server. With Mailfilter you can define your own filters (rules) to determine which e-mails should be delivered and which are considered waste. Requirements
PPC, Mac OS X 10.1 or later, Mailfilter.