Security problem

[LA]

I have horde 1.2.0 + IMP 2.2

My users in ther classrom can read another mail from another users,
i read the FAQ mailing lists about this problem, in the message

http://marc.theaimsgroup.com/?l=imp&m=96632280530027&w=2

there is a solution, I have probe but it's not enought what i have to do?

Thanks....