I think the question is: a link with the appearance of a button, like a submit button in a html page.
of a html element is for use in a html-browser. But not all email visitors use "browser-mail" (as Google-mail), some use e-mail programs (like Outlook, Thunderbird). E-mail programs aren't browsers, and have a bad html support.
Often the e-mail client doesn't show images, so I think an image-button is not reliable.
You can try a "[[U]Bulletproof Email Button[/U]", or the "[URL="http://www.industrydive.com/blog/how-to-make-html-email-buttons-that-rock/"][U]How to make HTML email buttons that rock[/U]](http://www.developerdrive.com/2012/05/creating-bulletproof-email-buttons/)", which has the same principle: an old-fashioned table model with a styled link in a cell.
If you follow them with a more html button-like design, you'll get something like this: