I have been using kerio mailserver for about 3 months now, and have been very happy with the results. The user interface is very simple, and the webmail is the first i've seen that really rivals exchange's webmail. For the price, features, and ease of use, this program is right on. The spam filter has been very good, the outlook integration is almost seamless, and the simplicity of the admin interface makes it simple to implement on any platform. Kerio's support team has been wonderful so far, walking me through any issues i had, from setup to bugfixes. I am using this with about 80 users, and they are happier with this then they were with exchange 5.