That's certainly a comprehensive answer.
When you say "it worked fine on my server", which does sound very much like something my host company would say but I have a lot more faith in you than them, do you mean that you transferred the corresponding pages to your servers, tested the same scenario there, and it found no issues with it? Even despite the changes you had to make to mail_php.php?
Sorry, can you clarify where the phpmailer directory comes in to things because that is not something that's generated by the WebAssist extension. Also, you didn't put it in the webassist folder, you just put it in the root, so I'm not sure of it's purpose in connection with the extension.
The reason I query the WebAssist script is because the host company provided a similar, albeit more basic, script and there were no problems with it. It was able to accept and, most importantly, send the data even if the customer had entered a Yahoo, Hotmail or AOL address as their e-mail address.
That leaves us in the following position...
1) The Fasthosts script encountered no problems sending.
2) The WebAssist script couldn't send when a Yahoo, Hotmail or AOL address was entered by the user.
I'd love to be harsh on our host, and they have made mistakes in the past, but that seems quite clear in terms of investigation/conclusion (the science bit!).
I agree with you that, due to the fact that the WebAssist e-mail code was able to send other e-mail addresses, that it confirmed that the script itself, the code syntax as you put it, was working fine, however where that conclusion fails is in the fact that it is/was the WebAssist code, alone, that failed with the Yahoo, Hotmail and AOL addresses.
Whatever you've done/changed, has certainly solved the issue but what is a pressing issue now is the testing of all the other forms we have that use the WebAssist extension. That is a job for tomorrow and, perhaps, that might further help with the investigation/conclusion. I'll keep you posted on that but I am aware that this affected another of our clients, so much so that we had to remove the e-mail field from the form!
I don't think this is something that you should just write off as being something that a host company has simply blown off. There is more to it and it's implications are quite far reaching.
I'll post back once I've checked other domains/clients, most of whom happen to be hosted with the same host so my confidence is low at this point.