My husband & I figured this out: If I've got a user with a email of the form email@example.com, My ISP isn't going to let me slap that into the "From:" field of an outgoing message even if firstname.lastname@example.org is originating this message. Smells too much like something illicit is going on. I have to use some dummy local address like email@example.com for the From field. I can CC firstname.lastname@example.org or even maybe use "reply_to:" (if I can figure out where that gets set).
As a show secretary, I'd really like it if my entrant's email address was the From, but I can manage without that. That email address is 2 other places in the form.