One would assume that administrators (Domain Admins and Enterprise Admins) would be allowed to fully control user mailboxes. Unfortunately, this presumption is shown to be incorrect when admins try to add additional mailboxes to their Outlook client. Of course, you can always manually set permissions on a per mailbox basis, but that defeats the purpose of global mailbox management. The cause is due to Microsoft deciding to globally set Deny permissions to Full Mailbox Access (Send As / Receive As) and hide the security tab in which one could edit these permission settings in Exchange System Manager. Fortunately, there is a simple registry fix for this problem.
1.Click Start, point to Run, and then type regedit.
2.Add registry key ShowSecurityPage
1.Go to HKEY_Current_UserSoftwareExchangeExAdmin
2.Once you reach the above section of the registry you need to create a DWORD called ShowSecurityPage.
3.A value of 1 (Numeric one) means on (show security tab), whilst 0 (Zero) means off.
4.Close the registry editor.
5.Close the Exchange System Manger, then reopen (no need for a reboot)
6.Right click on YourOrganization (Exchange), then click Properties.
7.Click the Security tab, then highlight the Domain Admins group.
8.Scroll down the permissions list and uncheck Deny for Send As and Receive As
9.Repeat the above step for the Enterprise Admins group.