Not every application is well enough behaved to call xdg-email for mailto: links, and not everyone is using a DE that will handle this. In these cases, xdg-open should detect the mailto: and pass it on to xdg-email.
e.g., in my own xdg-open I have added
> if (echo "$1" | grep -q '^mailto:')
> then xdg-email "$1"
to the open_generic routine, after the file: check and before passing it to the browser. I had a circumstance where clicking a mailto: on a web page was opening a new blank browser window instead of the intended behavior.