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.
Funnily enough, you've almost described the plot of the movie Sleep Dealer.
https://en.m.wikipedia.org/wiki/Sleep_Dealer