Use a reputable host, you may have to pay more than a few dollars a month. I've dealt with numerous spamhaus complaints, they generally result from idiotic users who send messages to spamhaus instead of clicking unsubscribe. If you spend an hour creating a really detailed form letter response it makes the AUP tickets go away quickly. When I sign up for hosting I detail exactly what we do and pay appropriately, most 'cheap' hosting places exist solely to pick up the remainder of the months service fee from a dubious spam complaint. If you spend $100 - $200 per month it's pretty easy to find a hosting provider that will let you run a single opt-in list, especially if you detail this up front in writing and refer to this in your response to any spam complaints. Web marketing shouldn't be a problem for any real hosting provider, unless you have extremely dynamic load I'm not sure why you'd bother with heroku. It's only a couple hours work to setup your own infrastructure. A quad-core server for $130 a month will run circles around what heroku provides for $130 per month.