After extensive testing and settings changes I can confirm that no e-mails are received when any of the following domain extensions are entered as the e-mail address by the user:
I changed the destination e-mail address to a BT address thinking that it was Fasthosts that was blocking submissions from the above domains but it turns out that BT are doing exactly the same thing. This makes me suspect that it's nothing to do with the destination address.
I've removed the SMTP server reference altogether - same problem.
The e-mails have the correct FROM and TO.
What now? :(