I have looked all through the forums and have thus resorted to actually posting a question 
Is there a good solution for managing an “opt out” list of internet email addresses that could conceivably grow to be very large and have the mail router use the list to filter outbound internet email?
I was hoping to avoid modifying the mail template. I would prefer that this run on the SMTP gateway server only.
Thanks in advance,
Subject: Can-Spam (Attn: Matt Chant)
I think that you will need a 3rd party tool to do this, such as MIMESweeper, Trend, CipherTrust, or something along those lines.
HTH.
Gregg
Subject: RE: Can-Spam (Attn: Matt Chant)
Don’t forget http://www.mimeshield.com/mimeshield
Subject: Can-Spam (Possible solution found)
I was able to create a hacked server side rule that includes an @DBLookup in the @IF side of the formula for the rule, while leaving the @DO side alone.
This works in my dev environment, but I’m worried about trying it out on a busy SMTP gateway.
I’ll post back when I have more data.