Just assure your host that you're not so stupid to have been using your own domain to be spamming someone else's shit. If your case is anything at all like what I was experiencing, you're getting hit with 1000-1300 e-mails an hour, might be worse if they're using a dirtier list. It might be somewhat taxing to relay all that on, but not so to send it to /dev/null. My tech admin wasn't concerned about the load in the least and it took him 2 minutes to do the reset.
The largest concern I had was a 1200 e-mail limit my ISP provides, if I didn't pull it all down at least once an hour, I started losing messages.
