Ask HN: Easiest way to autorespond to a new signup

I am about to launch my new startup and I am having a bit of trouble finding a dead simple way to automatically send out my beta .apk to anyone who submits their email on my site.

I have had limited success with Mail Chimp, I can get it to autorespond, but I cant edit any of the message.

The ideal solution would be a text only reply saying thanks for trying us out with a link to the .apk

Any thoughts?