Sometimes I just don't understand the Internet. Out of the blue, without editing anything, the form is working again.
To say I'm confused would be an understatement. Our client methodically checks their contact form on a regular basis, as did we once they informed us that it was no longer working, and there was definitely a problem.
Without doing anything, the form now works. Completely baffling!
Thanks for looking into that anyway Jason. Sorry to have troubled you.