Outlook: Grant Send As permission (to allow sending email as someone else)

"Send As" allows one user to send an email as though it came from another user. The recipient will not be given any indication that the email was composed by someone other than the stated sender.
This option can only be granted by a system administrator.
Follow the procedure to grant users the ability to send as another:

1. Log onto the server running Exchange.
2. Run Active Directory Users and Computers.
3. Under the "View" menu ensure that "Advanced Features" is ticked.
4. Find the user's account that you want to be able to send as, and open up the account properties.
5. Select the "Security" tab.
6. Click [Add ...] (under "Group or user names") and add the user (users or group) that is to be granted permission to send-as this account.
7. For each account added, highlight the account under "Group or user names" and in the "Permissions for ..." window grant the account "Send As" permission.
8. Click [OK] to close the account properties dialog.