Thanks. I submitted the bug report here: https://feedbackassistant.apple.com/feedback/19553015

In case someone who works on icloud imap is reading this, there is one more thing I've noticed. The imap LANGUAGE command is advertised as a capability but it always fails like this: 27 LANGUAGE en-US 27 BAD Parse Error (took 0 ms) I've tried various other language code strings and even quoted the language code string and it makes no difference.

I don't know if the CONDSTORE/QRESYNC issue has been fix or if maybe I'm connecting now to a different icloud imap server, but the problem seems to be resolved! I didn't note the version of the original server when I saw the problem, but the revision returned by imap ID is now "2204B190" and seems to work correctly. I still need to do more testing to make completely sure. There is still one issue that is not really CS/QR specific. I see this: 38 UID fetch 31:* (FLAGS) 38 OK FETCH completed (took 6 ms) According to RFC 3501, this should return the flags for the last UID message instead of returning nothing. See https://datatracker.ietf.org/doc/html/rfc3501#section-6.4.8: Also note that a UID range of 559:* always includes the UID of the last message in the mailbox, even if 559 is higher than any assigned UID value.