Here is a code snippet that shows how you can programmatically invoke the email client from the web client app:
procedure TForm1.WebButton1Click(Sender: TObject);
var
lnk: TJSHTMLElement;
begin
lnk := TJSHTMLElement(document.createElement('a'));
lnk['href'] := 'mailto:info@tmssoftware.com';
document.body.appendChild(lnk);
lnk.click;
lnk.parentElement.removeChild(lnk);
end;