what program should I use for this?
 

I've set up postfix to use virtual mailboxes and maildrop using mysql. How can I, or what program should I use to set up block lists so that users can specify domains from which they don't want to receive mail.

Couldn't they just set up rules in their own email client of their choice to do this? I would leave it up to them on that aspect instead of running a program of any such on your server per se.

If their using microsoft outlook, for instance, how could they do that?

Tools > Rules Wizard

They'll need to create rules to either delete, move to another folder, etc.

That sounds good. I will pitch that idea (I can't believe I didn't think of that).

None the less, they are gonna want to know how to block certain emails from notoriously bad domains, for the whole company.:
sasha@xfpsrfx27x.com // "This is obviously spam, we just want it gone from everywhere"

Should I use maildrop for that?