Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

uses Registry;

 procedure OpenMailClient;
 const
   cMailClient = '\SOFTWARE\Clients\Mail\';
 var
   reg: TRegistry;
   RegClientDefault: string;
 begin
   reg := TRegistry.Create;
   try
     with reg do
     begin
       CloseKey;
       RootKey := HKEY_LOCAL_MACHINE;
       if OpenKeyReadOnly(cMailClient) then  RegClientDefault := reg.ReadString('');
       if OpenKeyReadOnly(cMailClient + RegClientDefault + '\shell\open\command') then
         WinExec(PChar(ReadString('')), SW_SHOWNORMAL);
     end;
   finally
     reg.Free;
   end;
 end;

 procedure TForm1.Button1Click(Sender: TObject);
 begin
   OpenMailClient;
 end;