>I need to set and retrieve the default eMail client...

You can read registry information using API functions:

RegOpenKey(HKEY_CLASSES_ROOT, [\mailto\shell\open\command], @lnRegKey)
RegQueryValueEx(lnRegKey, 0, 0, @lnDataType, @lcBuffer, @lnBufSize)
RegCloseKey(lnRegKey)