We have used/sold:
Kerio - once they were bought out, and fired dev team (years ago) the product became a nightmare.
Smartermail - expensive, some bugs, but works well and interface is awesome
Icewarp -excellent product, we used/sold at same time as Kerio (2004-2012 time-frame) - If you need contact just try again, we know their team personally. Lot of government agencies still utilize that product.
Kolab - good product, can be pricey, and setup can be tough... This is what underlies Plesk Premium Email. We like Kolab since it is basically a good set of extensions to RoundCube, but again, setup can be a pain
SOGO - good overall product, self-hosted, easy to setup, some bugs with Outlook, some think the interface is "old" though I think it is classic and well-designed.
If I were to have to choose to provide premium email today, I would probably use SmarterMail or Icewarp in that order. We no longer sell premium email services, so no dog in that fight, but those two have great support, and are updated regularly.
If you want open source, SOGO is a good choice, but be ready to work at setup.
