All I've been looking to find an answer for this, but can't anywhere. From what I understand, in Exchange 2007, the Mailbox servers send ALL mail to Hub Transports, even if messages are being sent between users on the same store. This implies there is a queue on the Mailbox servers for messages to Hub Transports. If I go to Queue Viewer in the Exchange Management Console, however, I can only see queues for Hubs. Does anyone know if I can see the corresponding queue for Mailbox servers?

There is no such queue on the MB servers. From: http://www.msexchange.org/tutorials/Exchange-Server-2007-Email-Routing-Part1.html Store Driver Messages sent by a mailbox user enter the transport pipeline when they reach the sender’s outbox. The store driver on the Hub Transport retrieves it from the user’s Outbox and then transfers it to the submission queue. After the message has been successfully added to the submission queue, it is moved from the sender’s Outbox to the sender’s Sent Items. Messages are stored in MAPI format and must be converted to Summary Transport Neutral Encapsulation Format (S/TNEF) before being placed in the Submission Queue. This conversion is the job of the store driver, too. If this conversion is unsuccessful, a non-delivery report (NDR) is generated. Microsoft Exchange Mail Submission Service The Microsoft Exchange Mail Submission Service is a notification service that runs on Mailbox server roles. It notifies the Hub Transport server role to pick up the message from the sender’s Outbox. If there are multiple Hub Transport server roles on one Active Directory site, the Message Exchange Mail Submission service attempts to evenly distribute notifications between each transport role using static load balancing.Tim Harrington | MVP: Exchange | MCITP: EMA 2007/2010, MCITP: Server 2008, MCTS: OCS | Blog: http://HowDoUC.blogspot.com | Twitter: @twharrington