The genealogically related 'notmuch' can do that: by default it syncs certain notmuch tags (like read/unread) with Maildir flags, and then you can sync Maildir

The genealogically related 'notmuch' can do that: by default it syncs certain notmuch tags (like read/unread) with Maildir flags, and then you can sync Maildir with IMAP using something like offlineimap.