The only guaranteed way to do this is to use server-side code (PHP, JSP, ASP, et al.) to send the email.
*SOME* browsers will be correctly connected to a user's email client so you can do it from the browser, but likely that is around 1/3 to 1/2 of all users. On top of that, the email clients often issue what look like dire warnings to the users, telling that their email address will be revealed, etc., etc., if they proceed.
An optimist sees the glass as half full.
A pessimist sees the glass as half empty.
A realist drinks it no matter how much there is.