To use the Exchange Management Console to grant a user the Send As permission for another user's mailbox

1. Start the Exchange Management Console.
   
2. In the console tree, click Recipient Configuration.
   
3. In the result pane, select the mailbox for which you want to grant the Send As permission.
   
4. In the action pane, under the mailbox name, click Manage Send As Permission. The Manage Send As Permission wizard opens.
   
5. On the Manage Send As Permission page, click Add.
   
6. In Select User or Group, select the user to which you want to grant the Send As permission, and then click OK.
   
7. Click Manage.
   
8. On the Completion page, the Summary states whether the Send As permission was successfully granted. The summary also displays the Exchange Management Shell command that was used to grant the Send As permission.
   
9. Click Finish.
   
   http://technet.microsoft.com/en-us/library/aa998291%28EXCHG.80%29.aspx