So we had a spam account PM a bunch of people (at least two) which generated email notifications for users that haven't visited in a long time. This is really unfortunate. I've pruned every user older than a month with 0 posts (~700) and removed PM privileges for new users. I'm also looking at upgrading PHPBB to get the anti-spam code up to date.
If you're an old user visiting because you received a spam email I'm really sorry, and it won't happen again.