It's easy from the one direction: There's nothing stopping instance hosts from also hosting email addresses that match user account names. It might even be a good idea for some instances, in keeping a trust relationship, if your Mastodon instance was also your email provider of choice.
From the other direction is tougher, because you have a many-to-many relationship to try to consider. It's a variation of the problems of OG OpenID and/or Mozilla Persona (RIP to both). If I want to use [email protected], how do you map that to a "main" instance if you can't ask gmail.com because Google doesn't currently care about WebFinger or ActivityPub? Do you use a central database somewhere? Does that defeat the goals of decentralization you were going for in the first place?
Well first and foremost, people usually don't want their email published everywhere. Especially in this decentralized context, that'd be a gold mine for spammers.
