It would sit along side your Dovecot server. The Switchboard core subscribes to events in a folder (using IMAP IDLE) and issues change events to connected workers.
so is this like zeromq for mails?
I can't give an unqualified yes, but Switchboard is similar to zeromq in that it fans out events [new emails] to subscribed processes [workers].
We actually thought about using zeromq, but went with a simpler solution for now, i.e. using websockets.