If the server doesn't have access to the content of emails, then it reverts to a featureless blob store:
Search isn't possible
Previews can't be calculated
If you lose your private key, we can't recover your email
Spam checking on content isn't possible
To access mail on multiple devices, the private key needs to be shared securely between them
From: https://blog.fastmail.com/2016/12/10/why-we-dont-offer-pgp/
What's the tradeoff?
If the server doesn't have access to the content of emails, then it reverts to a featureless blob store:
update: want->NEED