[YetAnotherNick]

If you do end to end encryption, you can't persist the mails. So when you logout or clear cache or change browser or email client your past mails will be lost, which drastically reduces the purpose of mail as a long term thing.

[ognarb]

You can persist end to end encrypted mails on the server just fine. The important thing is that the keys for encrypting/decrypting the mails are not stored on the server too.