I just gave up and disabled contact form 7 after a customer called and said he’d sent an email via the form, and it never arrived. I wonder how many contacts we’ve lost over the past months or years from people who are not contacts, who tried reaching us, got a successful send message, but the message never arrived! I applaud your efforts to figure this out and devise a fix, but I’m kind of in shock that something we installed years ago has in all likelihood been a HUGE negative businesswise.