As explained above, you cannot reliably use mailto: links. You can try any of the Windows-specific methods;
  1. Simple Mapi (does not support HTML tags in the body)
  2. Extended Mapi (Does support HTML tags)
  3. CMC (Common Messaging Calls) Is a non-platform specific. When it is used on Windows it will access the MAPI properties.
  4. MAPI Activex
  5. CDO (Collaboration Data Objects).
  6. Automating Outlook
  7. and probably others

I would guess that CDO is worth the most investigation.