I've simply used this javascript code (using jquery but it's not strictly necessary) :
$( "#button" ).on( "click", function(event) {
$(this).attr('href', 'mailto:[email protected]?subject=hello');
});
When users click on the link, we replace the href attribute of the clicked element.
Be careful don't prevent the default comportment (event.preventDefault), we must let do it because we have just replaced the href where to go
I think robots can't see it, the address is protected from spams.