Problem solved, although not the way I would have liked. I’m now using Apple Mail to pull in my Northeastern mail (via POP) and Gmail (via IMAP) separately. I’m able to use Northeastern’s SMTP server off-campus as well as on. So all of my outgoing NU mail contains official-looking header information, and will thus not be intercepted by anyone’s spam filter.
Oh, well. Apple Mail’s not so bad, I suppose.