From a look at the xdg-email man page, it looks like it maps fairly well to the org.freedesktop.portal.Email.ComposeEmail method call.
Given that we want to switch the sandbox xdg-open over to preferring the xdg-desktop-portal API over userd, I think an xdg-email proxy should target the portal API exclusively rather than add a new userd API.
From a look at the xdg-email man page, it looks like it maps fairly well to the org.freedesktop .portal. Email.ComposeEm ail method call.
Given that we want to switch the sandbox xdg-open over to preferring the xdg-desktop-portal API over userd, I think an xdg-email proxy should target the portal API exclusively rather than add a new userd API.