We are currently using Atmail for our companies. Works well, support is tough as most of it is in Australia and can be slow.
Looking more at opensource using IRM and maybe http://www.mailpiler.org/en/index.html for archiving along with some RoundCube plugins to fill some gaps.
However,Atmail released 7.1.1 today which has some neat Admin reporting functions for users. As an Admin I can see a Summary, Account Stats (logins/protocol/time last, top senders, top receivers, etc) Graphs will full reporting for logins, etc, and even a Map that maps their IP logins...kinda handy.
http://demo.atmail.com/index.php/admin/index/login#
Check it out yourself. Clean interface now too. But you will pay for the prettiness.
My suggestion would be some strong backend user reporting for admin, from a business perspective it sure is nice to have.