[dfrussell]

Most likely, I would configure it to drop the email. Note that no DNS query is generated. However, if you wanted to, I'm sure you could configure it to do a DNS query but that would not be my recommended solution. Perhaps it could be allowed through but marked as possible spam?

In general, dropping anything except a virus is a bad idea and will eventually cause grief for the admin.

If inbound email relays are configured to simply refuse email from invalid domains, afterwards, it becomes a problem for the intermediate relay... which should not have accepted it anyway, so no big deal.

That said, maintaining a local domain name blacklist to circumvent a lookup is not a workable solution due to the frequency at which bogus domains are sent/changed... even the domain names included in spam are now changing rather frequently and they have to resolve.

For spam control, you want to nibble away at the content via multiple filters rather than trying to get it all at one time.

[taxcontrol]

In general, dropping anything except a virus is a bad idea and will eventually cause grief for the admin.

I've never had a problem with it. Usually all I need to do is provide the text out of the security policy that tells me to drop spam.