Deciding is the most time-consuming task when choosing an email marketing software or service. There are some top email marketing companies out there, each with its own features, prices and policies, and one must go through the process of testing them all before going with one of them. Because once you start using an email marketing software, chances are you'll be using it for a while.
Some of the best email marketings softwares are: iContact, ConstantContact, aWeber, Mailchimp, Getresponse, Vertical Response, PocketCM. I've wrote a while back an article comparing iContact with ConstantContact, if anyone else has any experiences with these email marketing services feel free to add your comment.
To read the comparison article I wrote go here: iContact vs ConstantContact.