[Dovecot] Numpty question about EXISTS...

[Steve [Dovecot]]

I'm fiddling with Ruby and its IMAP class.  I'm trying to write 
something which (at this stage) can be thought of as a variant on xbiff 
which doesn't need poll the server to establish the presence of new 
mail... I'd like to establish a notification when the contents of a 
folder changes... which is likely to be when a different IMAP client 
moves a message into a new folder.

I've been lead to believe that I can use the "EXISTS" response to 
trigger the 'new message present in IMAP folder" event in my program... 
but I don't seem to receive them when I would expect to when connected 
to my Dovecot server.  I get one "EXISTS" message when I call "select" - 
but that's it - it doesn't seem to notice when another client moves 
messages in/out of the folder.  Are my assumptions about IMAP incorrect 
- i.e. must I still poll?  Can Dovecot be configured to send EXIST 
responses (or any other form of notification) when messages are moved 
into a folder?

Any hints appreciated.

Steve