the form itself is easy, it’s the bot detection and spam prevention that’s hard. on my own sites, i’ve given-in and use the highest-level recaptcha, a hidden form field triggered by bots but not humans, and a server-side script for the mailing that also has some spam detection routines. they still get through, but far less often than a naked form would.

if you’re satisfied with your existing comments function, can you simply enable comments on your ‘contact’ page and hide them from public view?