Giving these Send As permissions is not possible with the Exchange Control Panel. We will use the Powershell Command Line interface to set these permissions.
Here is an example:
User1@domain.com needs to send emails as firstname.lastname@example.org.
Enter this command:
Add-RecipientPermission email@example.com -AccessRights SendAs -Trustee firstname.lastname@example.org
To see all the Send As permissions of a user:
Get-RecipientPermission - Trustee email@example.com
More information here : http://126.96.36.199/en-us/140/ff852815.aspx