It has been a blocker for many businesses to run mail hosting on Plesk 10 since its release in 2010. The core issue was that each User of Plesk could see a list of others - though good for Users of the same organization, it is not the case for mail hosting:
That has been solved in Plesk 10.2 - see
http://kb.parallels.com/en/111131 for how to eliminate "Users" tab for mail users. Since "Users" tab is hidden now, a password change can be accessed in user profile (click by user's login name in the top bar):
