Timo,
I'm working with a webmail client that periodically polls unread message counts for a list of folders. It currently does this by doing a LIST or LSUB and then iterating across all of the folders, running a SEARCH ALL UNSEEN, and counting the resulting UID list.
Eventually I'd like to see it using RFC5819 LIST-EXTENDED, but that requires a fair bit of work. In the mean time I'm trying to speed up the existing iteration. I've got it working using 'STATUS "mailbox" (UNSEEN)', but the language in RFC3501 suggest that this may be slow. There is a counterproposal to use RFC4731 ESEARCH and do 'SELECT "MAILBOX"'; 'SEARCH RETURN (COUNT) UNSEEN'.
From an IMAP server perspective, which do you anticipate would be faster? From a client perspective it seems like STATUS would be better since it involves less round-trips to the server and less output parsing, but given the warnings in the RFCs there is concern that it is in fact be more expensive.
-Brad