What about having a live validation on the textbox, and once it goes over 2000 (or whatever the maximum threshold is) then display 'This email is too long to be completed in the browser, please <span class="launchEmailClientLink">launch what you have in your email client</span>
'
To which I'd have
.launchEmailClientLink {
cursor: pointer;
color: #00F;
}
and jQuery this into your onDomReady
$('.launchEmailClientLink').bind('click',sendMail);