That's because e-mail doesn't have "signature" as a concept. If you want some particular text to show up at the end of the body of an e-mail, then you should append that text to the body, just as you did there.
However it appears that your e-mail is formatted as HTML, based on the commented-out code. In which case your problem is merely that HTML treats new-line characters as spaces. If you want a line break you have to use <br> -- clearly you knew that already.