Email Servers should support client-configuration options which allow filtering of email by replying "Recipient Not Found" on this server. Then, I could add a filter allowing emails from known addresses, and everyone else would get a not-found error. Hopefully they would interpret this as the email address being wrong, and delete it from their records.

Either way, the server would not receive the data, which makes it better than a client-side filter.

These guys have a pretty good package. It won't do exactly what you are asking but when you set the spam filter to "Reject" messages, the sender gets a nasty 550 5.7.1 Access not allowed. Kind of like the time you tried to kiss that girl in the bar on New Years Eve and she slapped you.

I have one in semi-production right now with 10 domains on it. The anti-virus feature kicks ass and the anti-spam works well but needs a lot of tweaks to make it efficient.

A problem with Grok's idea might be that the return address is often fakes on those emails, so sending back a 404-type message wouldn't do anything useful.

At least the server won't have to do much work. It just tells the sending server to piss off.

What if they make the sending server = your server? Unless you filter messages coming from fellow @valhallalegends.com people, you will either have an infinite loop or something else will break.

It won't loop in that scenario. It will just give the sending account a 550 5.7.1 and stop. I threw a Sendmail box into what should have been an infinite loop once, but after 10 loops, it realized what was going on and stopped.

[quote author=Thing link=board=2;threadid=3307;start=0#msg26457 date=1067520018]
I threw a Sendmail box into what should have been an infinite loop once, but after 10 loops, it realized what was going on and stopped.
[/quote]

Thing has created AI?

No, if "He" realized it, it's AI. I call my toaster an it. "It burned my friggin' toast!"