Consider that if blockchain is used for emails, then your emails will be permanently stored on the blockchain. The emails will be completely public forever if the private keys are compromised. Decentralization prevents censorship but does not protect privacy.

Of course current email providers might store copies of all your emails even if you delete them, while emails on the blockchain will certainly be encrypted with your private key. But what is the advantage of storing emails on a blockchain? A better solution is to simply encrypt the content of your emails with OpenPGP.