To explicitly grant another account permission to send as a mailbox owner, open the Active Directory Users and Computers management console and follow these steps:
- On the View menu, make sure that the Advanced Features option is selected. If this option is not selected, the Security page will not be visible for User account objects.
- Open the properties of the user account that owns the mailbox.
- Click the Security tab.
- If the account is not already in the list of group or user names, add the account that should have the Send As permission for this user.
- In the Permissions box, click the Allow for the “Send As” permission for the appropriate account.
- Click OK.
- Restart the Microsoft Exchange Information Store service on the affected Exchange server.