[boneitis]

You could, but you're dropping the qualification of end-to-end encryption.

Brainstorms of a (mere) hobbyist:

Some might reason that that yields additional hardening to traditional TLS-enabled webmail applications.

On the other hand, that is more architectural design and work shifted away from the endpoints (and wasted, complex efforts with no added benefit if improperly implemented by the provider).

[boneitis]

One more brainstorm,

The provider can serve key escrow and still have the end-user application perform the encryption, which may or may not technically qualify. It certainly wouldn't fly without skepticism in a popular service/standard.

I haven't looked into it deeply enough to present a confident statement either way.