Exchange admins will check the queue’s in Exchange System Manager (ESM) on a regular basis. But what if one of these queue’s is filling, do you know where to begin to check? Here are a few tips which will maybey help you find the problem a little bit faster.

• Local delivery: Contains inbound messages for delivery to mailboxes on the Exchange server. If there is a problem, then this is most likely the store or antivirus.
• DSN Messages Pending submission: This folder contains Delivery Status Notifications awaiting delivery. Its primarily used for NDR’s – Non Delivery Reports.
  This could be antivirus or third party spam filtering software.
• Messages awaiting directory lookup : Contains inbound messages awaiting recipient lookup in Active Directory. If the queue is filled, this could be a problem with the Global Catalog, directory lookups, or third party software.
• Waiting to be routed SMTP/X400 : Contains outbound SMTP/X400 messages still waiting to be routed to their destination server, when it has been determined the message will be sent. If the queue si filling up, this could be a DNS or a problem with the linkstate information.
• Unreachable destination : The default time for routing recalculation is set to 15 minutes. When the mail gets placed in the unreachable destination queue, it will be there for at least 15 minutes. If you want to force recalculation you must restart the services -> Routing Engine, SMTP Service.
• Failed message retry queue – SMTP : Contains outbound messages which couldn’t be delivered to their destination but will be given another attempt.
• Messages pending submission – SMTP : Contains messages accepted by the SMTP virtual server, but hasn’t yet been processed.
• Messages queued for deferred delivery – SMTP : Contains messages queued for deferred delivery (later time).
  SMTP Small/Small/Medium/Pro – X400 : Contains messages which are going in and out of the mailboxes located in each respective mailstore store which currently exists on the Exchange server

For more information : How to use Queue Viewer to troubleshoot mail flow issues in Exchange Server 2003