Sorry to take so long getting back to you on this one. I'm still having problems with this issue and have narrowed it down to affecting Yahoo, AOL & Hotmail addresses.
When a user enters their e-mail address in one of the form fields on the contact page of our website, and that e-mail address is either Yahoo, AOL or Hotmail, the form does not arrive at destination. All other e-mail addresses work.
FTP details for the site are detailed below along with a link to the page/form affected.
The host, Fasthosts, sent us links to some advice they give on form scripts which we've tried to follow as best we can. I've created an SPF record, and have changed the smtp server address, but neither have resolved the problem.
Sending e-mail from your website
Using Fasthosts SMTP servers
Hoping you can help resolve this one. My conclusion is that it is either:
1) The WebAssist form script settings
2) Fasthosts filtering/blocking (for whatever reason)