The default mailto: client is the client that gets started when you click on a mailto:email-address link on a webpage.

To change the default mailto: client by hand:

• Open any folder and click on "Tools", which is one of the choices along the top.
  
• After clicking "Tools", select "Folder Options" and then "File Types".
  
• Under "File Types", scroll down to the "URL:MailTo Protocol" entry and select it.
  
• Click "Advanced", select "open" and then "Edit".
  
• Type "C\emmesmail\msv.exe -compose %1" in the "Application used to perform action" box and click OK.
  

Emmesmail is now your default mailto client and will open in compose mode whenever you click on a mailto: link.