No, that's not it at all.
If you go to the following web page:
...and you complete the form, one of the form fields asks for an e-mail address so that my client can respond to the customer enquiry.
If the customer enters a Yahoo, Hotmail or AOL address in that form field, very common e-mail address extensions, the form doesn't arrive. If you enter any other e-mail address, it arrives as expected.
The host has triple tested this with a form script that they use and all e-mail addresses are being allowed through, it is only the WebAssist form script that seems to be blocking e-mail from those extensions.
Hope you can help. FTP details are listed above.