This is the most complete and reliable email delivery solution for pros. It has a delivery API, as well as google email marketing apps for mailing campaigns (blasts) and autoresponders. The best? They don't limit the subscribers for your marketing email lists.

Easy setup, a real person for tech support

Ease of use. It's easy to set up using the standard SMTP parameters or their API hooks with several applications; I just use their wordpress plugin and I'm up and running in 5m or less.