Oy. Do NOT self-host an email autoresponder. I wrote a great comment about this a few years ago on Brian Armstrong's blog--and he's now doing Coinbase after deciding the email stuff wasn't worth it.
Look in my profile to see who I am and why I'd say this, but as a quick summary, I have years of experience and have also helped companies set up autoresponders that do over 1M emails a day, and that's exactly why I say this. It is a clusterf*ck and a rabbit hole a mile deep and a mile wide. Pay the money (or in Mailchimp's case, don't pay any money until you get bigger.) It is worth EVERY penny.
By the way, any good autoresponder (I use Mailchimp and Aweber and like them both for different purposes) will let you easily export/back up contacts, and none of the big companies are going anywhere.