After working with ESPs for years it's so refreshing to manage the MTA yourself. Obviously this isn't for everyone, but it can really help you optimize deliverability and reduce costs. Of course PowerMTA (Port 25) is a lot more powerful and probably performs better, but it's also a lot more expensive, doesn't have a seamlessly coupled front-end, and much more difficult to use. GreenArrow engine was a perfect fit for us and has served our needs perfectly. These are some of the aspects I like most: -A+ customer service. I've dealt with a lot of ESPs, and absolutely no one beats GreenArrow. They are extremely hardworking, ethical, truthful, and trustworthy. The culture there is really outstanding, and much different than other ESPs we have worked with where it's obvious they are just trying to milk us to meet quotas -The documentation is fantastic. Check out docs.drh.net. -Resources are also fantastic. Their ebooks are really well written, simple, to the point, and insightful. They aren't just rehashing simple points and wasting your time. You can really learn a lot by reading what they have to offer. -The UI for Engine is very easy to learn and surprisingly feature rich. You have a lot of good reporting for campaigns but also for time periods and for lists. -The GA seed testing for inbox monitoring works very well. Granted, all seed testing is inherently flawed, but it's as good as any I've tested