I have used IceWarp for email since 2002. 11 years later, I can honestly say that I remain a happy customer. I am a Microsoft Certified Exchange administrator, and as such, also maintain an exchange environment for a couple of businesses.
IceWarp is easy to install and maintain. It provides several enhancements over exchange. It is extremely easy when you compare it to the complexity of Microsoft Exchange. In fact, it is easy to add a new domain and be ready to go without the hassle, and without the licensing complexity.
I use this to host about 100 domains with somewhere around 300 email accounts. The software is up to date and maintained by the vendor.