Forget it; this might work with Outlook or maybe even GMail but you won't be able to get this working properly supporting most other E-mail clients out there (and there's a shitton of 'em).
You're better of using a simple PHP script (check out PHPMailer) or use a hosted solution (Google "email form hosted", "free email form hosting" or something similar)
By the way, you are looking for the term "Percent-encoding" (also called url-encoding and Javascript uses encodeUri/encodeUriComponent (make sure you understand the differences!)). You will need to encode a whole lot more than just newlines.