Just use more than one outbound mail server. All "normal" mail goes through a server that's rate-limited heavily -- a few dozen an hour, at the most. Bulk email has to be sent through a separate outbound mail server, and there can be much more scrutiny on what goes through that -- because the legitimate "mass mailings" are going to be comparatively rare, and are probably worth having someone take a look at them, to make sure they're OK.