Docpad Contact Form: docpad-plugin-formmail, docpad-plugin-contactform

Don't use the listed plugins.

Save yourself the time - I'm writing this post after spending almost 2 hours trying to finagle either of the plugins into working as I want it.

I'd suggest a backend-less form such as: https://formend.com/

(Edit: Removed Brace from the recommendations - their service wasn't working for me at all unfortunately. Will update if that situation changes - http://forms.brace.io/. For now, I'd suggest Formend, which is working perfectly.)