Exchange 2000 Send As permission

I'm using exchange 2000. In order to have the send as permission for user A on mailbox of user B, I go to AD, search for user B's user account and under security tab, add user A and selected the send as permission and restarted the information store. After half an hour, I realized that the user A is not in the list anymore. I thought that I didn't add into it. So I repeated the same step again and again and the same thing happen. Why will this thing happen? The user A will keep on disappear from the list and i need to add it manually. "Allow inheritable permissions from parent to propagate to this object" is not selected. Any help would be appreciated.