about access control

Hi,

Please help me on the following problem:

i have two users: A and B,
A is a domain user and B is a power user,
in Active directory user and computers snap in, i want to add user A to user
B's security settings and allow A to have send as permit. it works, but
after about 1 hour, A will disappear from B's securrity setting, and things
just want work anymore.
(i accessed the security tab by check the view--advanced features with
details checked)

anybody can tell me why? I am using SBS 2003 R2

many thanks in advance!
josh


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