Ok, Sendmail issue should be resolved now. More good news. The cause of the problem was that the forum was sending out over 2k notifications a day. That's a whole lot of people pming and watching topics! They've raised our quota, which should hold things for awhile but we may well need to outsource sendmail at some time in the not too distant future. So please keep an ear to the ground for a reliable, free or very inexpensive provider. When I say reliable, I mean technically stable, of course. More importantly, trustworthy with what may be sensitive information.
Thanks, ya'll, for all of your help and support!