In order to Grant "Send As" permissions to a user using Active directory MMC snap in you should:

1. Start < Run< type dsa.msc and then press Enter on the domain controller.
2. Go to menu < click View <Advanced Features.
3. Open the user Properties for the requested user.
4. Click the Security tab < click ADD to select the user to be granted "Send As" permission < click OK.
5. Select the added user in the user’s box below < mark Allow for the "Send As" permission in the permissions list.