Protonmail is encrypted indeed, but is not free (has a limited option free) it's stored on a private server, with private owners, with commercials and you not earn nothing.
Soverin it's not encrypted, also private, also with a server own by someone and you have to pay.
Tutanota it's encrypted, it's not free (more features/storage more money to pay), this to has private server and you don't earn a thing using them... in spite of all the commercials that they all run.
A Block-Chain e-mail should be one by community (not by one ore few person that decide), with your private files spread in tiny bits in all the network (not a server with all your data), the level of encryption in a block-chain it's far superior and the best of all should pay you for using it.
Soverin is PGP encrypted the dutch www.bof.nl (bits of freedom) chose their service to provide secure e-mail
Tutanota is also available in a gratis option, but the space is then a bit limited.
I see how decentralised is better, but i have not found one yet that i would actually use.
I'll try to come back to this article to find more options.
Ok, thanks and good luck! :)