I’ve been using Atom Park software for a couple years now and as a freelance web developer this is one of the most valuable software vendors  I currently use. I own Atomic Email Sender and Atomic Email Hunter. What I consider the key points are:

• Easiness to use, just plain-none-complicated instructions
• Many settings to adapt to every situation.
• Although I only had to use their support service one time I can say their support team is very responsive and knowledgeable.

I’m still due to upgrade to the Atomic Email Studio, which after witnessing the wonders of their other software, I’m going to do it very soon.