Yeah, "Bring your own domain" is a super cool feature idea. We could even re-use Acme HTTP-01 or DNS-01 challenges to verify the domain.

i would like this too. i think you can do it with apple email relay.

currently i have example.com as my email, which i use with mailbox.org, would it be possible to keep using it with mailbox.org and then for mozilla to allow it to be used for email relays? e.g.

name@example.com goes to mailbox.org

3859dhtog@example.com goes to firefox relay

(not currently ofc, but a future thing)

I assume this means you have an MX record at example.com pointing to your/mailbox.org SMTP server? AIUI, a sending MTA will look up the MX record for example.com by preference order and will deliver emails to the first server that accepts the connection.

So it may depend if you can configure your mailbox.org account/server to reject connections from servers trying to send mail to unknown addresses? Then the sending MTA server might "fail over" to the Relay server instead?