Comment 5 for bug 1863625

One thing that would be worth trying from the sandbox side: the flatpak-xdg-utils package contains an implementation of xdg-email that calls into xdg-desktop-portal. Any xdg-email proxy provided to snaps would look very similar to this.

Flatpak-xdg-utils is not packaged in xenial or bionic, but it looks like it would be fairly simple to build from source within e.g. the Chromium snap:

https://github.com/flatpak/flatpak-xdg-utils