Hello. A user of ours recently pointed this bug out out. It wouldn’t be too big of a deal except that his web host (Lunar Pages) actually flags his Discourse emails as suspicious because of this and perhaps other email/hosts might as well.
Discourse properly turns that into a “mailto” link on the forum itself. But when that post appears in a Discourse email, the underlying html is: <a href="https://www.forumurl.commailto:username@domain.com">username@domain.com</a>
I can confirm this (sorry for the delay) - the HTML definitely appears broken in the raw email. This affects standard email notifications (watching a topic, mentioned, etc) in addition to the emails mentioned above.