One thing that one must not forget is also integration with your existing website. If your running on Joomla, WordPress, Drupal or such likes, better you find how the user integration will go with your CMS and one of those email services. You don’t want the client to have to register for your autoresponder on one form and then have to register a second time for your website! You may find that plug-ins to do such things might restrict your choice, check it out first or consult with your webmaster!
Before you can send AWesome emails with AWeber, you'll need someone to send it to. The easiest way to begin building your list is to import contacts that have opted-in to your email list, or gave you permission to send them emails. These could be subscribers you've emailed from another provider, or a list of contacts that you've built through your business.