A recent testimonial - Steve Williams' October 20th, 2006 message to the OpenBSD-misc mailing list:
Mcafee Webshield protecting a MS Exchange Server
daily load: "If we received 10,000 emails, our Webshield would have trapped over 20,000 spam"
Then a virus appeared which the Webshield did not handle.
Solution: put an OpenBSD box with spamd greylisting in front