Intranet messages through Outlook

On our Intranet we have a message screen which allows users to select a user and write a short message. When they click send it sends the message through their Outlook, quick and easy.

On one machine however, this has stopped working correctly. Im sure the user has ticked a box or set an option in Outlook, so when they click send the ‘new message’ screen from outlook appears instead with the correct email address in, but looses the message. Ofcourse this is the machine that uses the message screen most often.

What could be causing this?