Security is the most important issue.
Merak is SO good, we're kind of embarrassed to admit we were skeptical of using it, and only gave it a go because of the great price. We're not a big company, but we're confident that Merak will grow with our needs. We've barely explored half the options Merak has to offer it's so fully featured. And if something is missing, there's always a willing programmer the other side. No matter what your mail requirements, you don't need to look further than Merak.