uses
  ShellApi;


procedure TForm1.Button1Click(Sender: TObject);
var
  strEmail, strSubject, strBody, Param: string;
begin
  strEmail := 'user@host.com';
  strSubject := 'Your Subject';
  strBody := 'Your Message Text';
  Param := 'mailto:' + strEmail + '?subject=' + strSubject +
           '&Body=' + strBody;
  ShellExecute(Form1.Handle, 'open', PChar(Param), nil, nil, SW_SHOWNORMAL);
end;