I am not really sure what you are talking about...a email account consists of either emails you received or emails you sent.
So naturally for all your emails you have in your inbox the sender has them in their outbox...and every email you sent also exists as a copy in somebody else’s inbox.
And you can’t control whether the other parties host their own email server or if they use a hosted service...in which case, guess what: now a third party also has a copy.
Now to hosting your own email server...who is gonna stop your Datacenter from obtaining a copy of your emails? They call it backups ;)
It’s easy to talk to your Barista about decentralization and blockchain as the solution to privacy...but actually making that a reality is not.
And we haven’t even touched on spam...how you gonna fight that in a decentralized network? How about bots?
I could go on but I hope you get the picture.