There aren’t any that go direct to maildir. There is mujmap that syncs to notmuch. I wasn’t able to get it to work the way I wanted, and wrote my own that I’d be happy to share. I’ve been using it for around a month with no issues but that’s certainly not much confidence yet!

The advantage of going straight to notmuch is that the data model is so similar to JMAP that synchronizing is just that much easier. Both ends can give you a state and all emails which have changed from that state. Then it’s just a matter of updating the tags on those to each end or uploading/downloading the email if it’s new.

I get something like 1.7kB round trip data usage when there are no updates (really it’s 0kB save for the keepalive when used in daemon mode) and ~100ms for that round trip. This is in comparison to several megabytes for a single mbsync poll (even when there are 0 changes) and ~5 seconds