I “had” the same problem, and mail from may have solved but what I think was really going on is the mail was being sent from “noreply@websitename dot com” Most reputable hosts don’t allow mail to be sent from an account that does not exist. Once I created the account noreply everything worked as it should.
The new reg’s probably worked on the clean basic installs because the server could tell the email wasn’t be relayed, (the server saw it was coming from WP) once you install BP the host didn’t recognize who the mail was coming from.
