Mail would target specific relays (like your personal inbox relay) which would require AUTH to access, so yes.
Discussion
So I would have to host or rent a relay?
or just use a public one? AUTH works on relays today.
client handles auth, whether relays support it is a separate matter, and for what reason
it can be added to strfry with a plugin, my relay replicatr handles it natively, uses it to auth users for a DM based CLI, and has a fallback in chat that asks the user to paste a challenge string (which includes a timestamp)
which is why you all client devs better get your NIP-42 house in order
i've been bangin this drum now for over a month, but maybe you'll listen when noobs come and you are reminded they might be turned off by metadata leaks!