It seems to be an unrelated code base.

that's fine (or could be :-). I am thinking more about syncing the state of differnt mailboxes, local storage on mobile versus desktop, what is read / unread, user tags and other such things. I.e. how to use the mobile and desktop together as different clients to the same email content.

Read/unread and tags are all supported by IMAP already so that should be possible for sure. Same with starring/flagging email, any willing mail client could implement those.

The big integration points I see are within special features such as some kind of cross device PGP support, the ability to sync customised "from" addresses, and other features that the more basic built-in email apps lack.

Perhaps the code could be further extended to also support calendar, task, and chat functionality, just like on desktop, but that would take more effort.

The majority of what you are talking about (read / unread, user tags and other such things) is handled by the IMAP server. eg; I routinely start drafts on mobile, save it, then continue to draft on a computer before sending. Also set flags on mobile for follow up later on desktop. Almost none of this depends on the client itself, it's all stored in IMAP.

Late addition but yes of course the apps must support those IMAP features. They work hand in hand for this functionality.

Presently if you aren't able to do any of the above something is not configured correctly. Some apps have differing opinions on where the sent/drafts/deleted folders should be for example.

In typical use both are IMAP clients.

They plan to integrate Firefox Sync with Thunderbird (android and desktop)[1]

[1]: https://developer.thunderbird.net/planning/roadmap#firefox-s...

that would actually be... quite good. but I'd love to see more ambition (and community support) for thunderbird. I would like it (or some future converge) to be my RSS reader and, why not, my fediverse client.