holding incoming messages?

I'm wanting to do some serious hacking on my mail setup. To make
life easier, I'd like to be able to arrange that mail to specific
destinations (either host or address, as I need to) be frozen
before attempting delivery. I could, of course, put everything on
hold, by putting "freeze" into the system filter file. However,
I'd like to do something a little more selective. :-) Suggestions?