Hallo Steffi,
ich hatte auch das Problem und habe mir dafür folgende Beschreibung gesichert. Gilt in diesem Fall für Windows OS.
Tommy
1. In My Computer, click Folder Options on the View menu, and then click File Types.
2. Click the URL:MailToProtocol line to select it, click Edit, click Open in the Actions box, and then click Edit.
The command line that controls the program that is used to open a MailTo Uniform Resource Locator (URL) appears in the "Application used to perform action" box.
You may have something like the following:
rundll32.exe url.dll,MailToProtocolHandler %l
OR
"C:\Program Files\Microsoft Office\Office\OUTLOOK.EXE" -c IPM.Note
3. Replace the current command line with the following:
C:\directory\notes.exe =C:\directory\notes.ini %1
Note: This entry has a limitation of 256 characters set by the operating system, meaning that a very long URL:Mailto will be cut.
4. Click OK until you return to My Computer.