I've been using AcyMailing for several months after looking at many other mass mail systems and trying several (including commercial ones).
AcyMailing is hands down THE BEST!
The starter version is FREE and it's loaded with far more features than most commercial versions!
It's VERY customizable (e.g. making a template that matches your site, or several templates for different lists) and configurable (e.g. throttling according to your host's requirements). Some knowledge of HTML will help when customizing templates, but they give you some good tools to work with.
Some people will find that there's a "learning curve" figuring everything out. (Full disclosure - I didn't read much of the documentation; I'm a "hands on" type. They do have lots of documentation and tutorials, and include "Help" information on every page.) Let's face it: Mass mail customized to your particular needs takes some effort on your part. Believe me, the results are worth the effort.
It installs quickly and flawlessly - including updates.
Their service and support are awesome! When I first started using it, the unsubscribe page seemed confusing. I contacted them and they quickly sent a temporary fix; then included that in the next release. I recently requested a new feature - not knowing how hard it might be to implement. The staff quickly responded that it was a good feature to add and that they would do so. That was only a few days ago and it's in the new release today!
AcyMailing statistics are amazing! You can easily find out not only how many people opened your mail, but who and when. Where else can you get that kind of information without paying a hefty price?
AcyMailing Starter is PROFESSIONAL all the way (except you don't have to pay the professional price).
Thank you, AcyMailing Team! 5+ stars!