BuddyPress.org

The inner workings of the e-mail spam detectors are strange and mysterious. You are probably right in that having your mail sent by a server on a domain hostname different than the domain specified in the “from” field in the mail (your domain) counts against them. I’d set up a big “check your junk mail” blurb as suggested above, and recommend your members to mark all BuddyPress e-mails as “not spam”.