Why not select all the interesting mailboxes to poll (in different sessions) and NOOP them or use IDLE extension.
Because this creates an enourmous overhead in connections, server workload etc. Let's assume we have only 10 mailboxes "shared" with a workgroup of 20 users, you'd have 200 open connections on the server for simple new-mail-notification?! And these are small numbers...
TCP connections are cheap, once opened. And even opening can be cheap, depending on whether you're using TLS or not and what sort of authentication you're using.
A worse problem is that a client can use this method only if it knows the server supports concurrent access to all of the relevant mailboxes. If the server doesn't, only one of the 20 users can observe new mail in each shared folder.
--Arnt
