Ok, there is a mystery afoot.

I have other php "contact us" forms which function perfectly, but the "mail.php" send mail test isn't working for any of my domains. I'll contact tech support and find out if there's an issue that needs to be resolved on their end.

The php contact us forms are working, just not this tester, which makes absolutely no sense whatsoever.

Thanks ever so much for such quick responses admin!