https://github.github.com/gfm/#example-631
+ can occur before the @, but not after
......

hello@mail+xyz.example isn't valid, but hello+xyz@mail.example is.

......

<p data-sourcepos="1:1-1:66">hello@mail+xyz.example isn't valid, but <a data-sourcepos="1:41-1:62" href="mailto:hello+xyz@mail.example">hello+xyz@mail.example</a> is.</p>