The protocol has ossified and been entrenched. In general more efficient usage of IMAP relies on extensions to the protocol.
A modern replacement (JMAP) hasn't been adopted by major providers.
If you really cared about data transfer size you'd use something like Protobuf.
Well and you could of course server JMAP over something like Protobuf, JSON is used for ease of use but it's not like thats required feature.
The bigger benefit of JMAP is it's well designed for dealing with deltas and changes and syncing data requiring less roundtripping than IMAP.
Can confirm that (implemented JMAP). Deltas and asking for the entire mailbox instead of folder by folder is really good. And as a side effect: 1. Less client-side logic for sync. 2. There are many JSON parsers compared to IMAP, making it way easier to use. For example, in C++, you only need a JSON headers-only library, whereas IMAP is meh—only Linux, or clunky usage. Btw, I made one from scratch (IMAP Parser). 3. Goodies like offloading HTTP to the mobile network stack, which supports TLS 1.3 and offline (background) sync, compared to manually extracting certificates from an IMAP connection to validate against the device keychain. It's just 10x easier.
It's designed well, from the ground up with a lot of lessons learned from some pretty serious email junkies. Anyone hemming and hawing about JSON being selected as the transport encoding or whatever is just raising silly points. You could do JMAP over protobuf too probably if you cared, but the whole point (IMO) is to get the hell away from unstructured protocols.
Yep, there's always someone ranting about JSON inefficiencies, which I don't observe at all :)
oh i mean i rant about it too but its like, a byproduct of the real goal, and since most clients are running some type of webapp it makes sense. you could definitely use something like messagepack or protobuf if you had an extension that negotiates it.
IMAP 4rev2 merged in a most of the common extensions into the base protocol. Sadly they left out the THREAD, QRESYNC and OBJECTID, though they reference them.
Of course, major providers like Gmail don't support that as well.
JMAP is fully supported (since 2019) for Fastmail
Jmap was developed by Fastmail, meanwhile all primary FLOSS mail servers do not support it and just new projects like Stalwart are starting to bring it up.