Hi, I am trying to give a user "Send As" permission on a mailbox. On the AD account, I go to the security tab and add user's account and give her "Send As"permission on the mailbox, but after 1hr or so, the user disappears from the security list. I have done this about 5x now and it comes happening. Can anyone help please. Thanks.

crank up auditing on your default domain controllers policy and check the security logs.

user cannot be part of protected group like "Domain Admins", removed user from the group, everything seems to be fine. Reference: http://support.microsoft.com/kb/907434