Offline is just a matter stable identifiers on the server. It's the client that is responsible for caching the e-mail content on the device. Also, the fact tha

Offline is just a matter stable identifiers on the server. It's the client that is responsible for caching the e-mail content on the device. Also, the fact that JMAP makes it convenient to only download part of a message, and let the server do all of the message parsing does not encourage the client developer to do much processing client-side and keep e-mails blob in cache to be processed offline.