HTML: Soft hyphen (­) without dash?

Though I'm not sure how this does cross-browser (probably pretty well), you could always use the thin space character ( ) or the zero-width space (​).⁺⁺

john.doe @example.com

⁺⁺ I would not suggest using the zero-width space, as apparently some browsers will not render it correctly (source).

Use a zero-width space: ​

john.doe​@example.com

In action here: http://jsfiddle.net/uTXwx/1/