SendGuard 

To transfer a license from one computer to another, you need to deactivate it on the first computer before it can be activated on the second.

To deactivate please follow the steps below:
From MS Outlook, select the Addins4Outlook menu.
Choose SendGuard.
Click About SendGuard
Click the Deactivate button to deactivate.

You will now be able to activate SendGuard on another computer.