Bingo.
Okay, first things first - go into the UBB's email settings, make sure that each and every option there has a selection (except the SMTP server name, as you're using sendmail) - make sure specifically that the choice between HTML and text email is made - that's when the error you gave occurs.
Question... when the UBB did send mail, did the user receive it FROM administrator@, or is that address only seen when UBB generates errors?
