There is no way of finding out, in client-side code running in the browser, what the end user's default mail client is and if there was, you'd still have to find a way to get the client-side code to communicate with that client. That is possible if you use ActiveX but that raises security issues and it only works on Windows PC's.
Mailto is the default way to work, and if 2000 characters is not enough, I'm afraid the only alternative is to have the server send the message. Of course the server may put any email address in the From:
field, so for the person receiving the message, there would be no difference. If needed, the server may put the "author" in Cc:
or Bcc:
.