I've had several clients ask this, and I've never been able to get or give a satisfactory answer. Can you (practically) put a working form into an HTML email?
My understanding is that you can, but that there's no guarantee of email clients properly handling it, so that method would only be practical with individual recipients that you'd had a chance to test with, rather than with huge mailing lists of people using diverse email clients. But my understanding comes from a lot of developer friends shrugging at the question and no authority that I've found giving a clear answer either way.
What does Stack Overflow think? What's the best answer when you're asked to do this by a client who, for instance, wants a few hundred people to fill out a survey?
(Don't bother just answering "provide a link to a form on a website". That's the obvious answer. I want to know specifically if there's a consensus about forms in email.)