Note that the key could be client side and the email service could encrypt the email in a way as to not find out your private key. However, and this is key, a

Note that the key could be client side and the email service could encrypt the email in a way as to not find out your private key. However, and this is key, a majority of email is sent and received unencrypted via their servers, because that's just how email works. In other words, email at rest might be encrypted, but most of the email in transit is not. This means that a man in the middle (e.g. the NSA) can still intercept all your communications. And even with PGP encryption, which is e2e, the meta data is still unencrypted because that's what the email protocol demands. Having the ability to see whom you're communicating with, that's enough in many cases. Email is simply not a good solution for e2e encryption. If you have secrets to protect, there are now other solutions available, like Signal and even Signal has some issues (like its reliance on phone numbers for identification).