Using SMTP Queues to Troubleshoot Message Flow


During message categorization and delivery, all mail is sent through the SMTP queues of an SMTP virtual server. If there is a problem delivering the message at any point in the process, the message remains in the queue where the problem occurred until the problem is remedied.

Use the SMTP queues to isolate possible causes of mail flow issues. If a queue is in a Retry status, in Queue Viewer, select the queue and check the properties of the queue to determine the cause. For example, if the queue properties display a message that is similar to "An SMTP error has occurred," review your server's event logs to locate any SMTP errors. If there are no events in the log, increase the SMTP logging level, by right-clicking the Exchange server, clicking Properties, clicking the Diagnostics Logging tab, and then selecting MSExchangeTransport.

The following table lists the SMTP queues, their descriptions, and troubleshooting information for message accumulation in each queue.

SMTP queues

Queue name Description Causes of message accumulation

DSN messages pending submission

Contains delivery status notifications, also known as non-delivery reports (NDRs), which are ready to be delivered by Exchange.


The following operations are unavailable for this queue: Delete All Messages (no NDR) and Delete All Messages (NDR).

Messages can accumulate in this queue if the store service is unavailable or not running, or if problems exist with the IMAIL Exchange store component, which is the store component that performs message conversion.

Check the event log for possible errors with the store service.

Failed message retry queue

Contains messages that Exchange did not deliver, but that the serve try to send again.


The following operations are unavailable for this queue: Delete All Messages (no NDR) and Delete All Messages (NDR).

Messages can accumulate in this queue if a problem exists with DNS or SMTP.

Check the event log to determine whether an SMTP problem exists. Verify your DNS configuration using NSlookup or another utility.

On rare occasions, a corrupted message can remain in this queue. To determine whether a message is corrupted, try to view its properties. If some properties are not accessible, this can indicate message corruption.

Messages queued for deferred delivery

Contains messages queued for delivery later, including messages sent by earlier versions of Outlook clients. (You can set this option in Outlook clients.)

Messages sent by earlier versions of Outlook treat deferred delivery slightly differently. Earlier versions of Outlook depend on the MTA for message delivery because SMTP, not the MTA, now handles message delivery.

These messages remain in this queue until their scheduled delivery time.

Possible causes of message accumulation include:

  • Messages are sent to a user's mailbox while the mailbox is being moved.

  • The user does not yet have a mailbox created, and no master account security identifier (SID) exists for the user. For more information, see Microsoft Knowledge Base Article 316047, "XADM: Addressing Problems That Are Created When You Enable ADC-Generated Accounts."

  • The message may be corrupted, or the recipient may not be valid.

To determine if a message is corrupted, verify its properties. If some properties are not accessible, this can indicate a corrupted message. Also, verify that the recipient is valid.

Local delivery

Contains messages that are queued on the Exchange server for local delivery to an Exchange mailbox.

Messages can accumulate in this queue if the Exchange server is not accepting messages for local delivery. Slow or sporadic message delivery can indicate a looping message or a performance problem.

This queue is affected by the Exchange store. Increase diagnostic logging for the Exchange store as described in "Configuring Diagnostic Logging for SMTP."

Messages awaiting directory lookup

Contains messages addressed to recipients who have not yet been resolved against Active Directory. Messages are also held here while distribution lists are expanded.

Typically, messages accumulate in this queue because the advanced queuing engine cannot categorize the message. The advanced queuing engine may not be able to access the global catalog servers and access recipient information, or the global catalog servers are unreachable or performing slowly.

The categorizer affects this queue. Increase diagnostic logging for the categorizer as described in "Configuring Diagnostic Logging for SMTP."

Messages waiting to be routed

Holds messages until their next-destination server is determined, and then moves them to their respective link queues.

Messages accumulate in this queue if Exchange routing problems exist. Message routing may be experiencing problems.

Increase diagnostic logging for routing as described in "Configuring Diagnostic Logging for SMTP."

[Connector name | Server name | Remote domain]

Holds messages destined for a remote delivery. The name of the queue matches the remote delivery destination, which may be a connector, a server, or a domain.

If messages accumulate in this queue, you must first identify the status of the queue. If the queue status is Retry, check the queue properties to determine the reason that it is in this state. For DNS issues, use Nslookup and Telnet to troubleshoot. If the host is unreachable, use Telnet to make sure that the remote server is responding.

Final destination currently unreachable

Contains messages for which the final destination server cannot be reached. For example, Exchange cannot determine a network path of the final destination.

Messages can accumulate in this queue if no route exists for delivery. Additionally, when a connector or a remote delivery queue is unavailable or in Retry for a while, and no alternative route exists to the connector or remote destination, new messages queue here. Messages can remain in this queue until an administrator fixes the problem or defines an alternative route. To get new messages to flow to their remote destination queue, helping you to force a connection and get a Network Monitor (NetMon) trace, restart the SMTP virtual server.


Holds messages that have been acknowledged and accepted by the SMTP service. The processing of these messages has not begun.

Messages that are accumulating constantly may indicate a performance problem. Occasional peaks in performance can cause messages to appear in this queue intermittently.

Message accumulation in this queue can also indicate problems with a custom event sink or a third-party event sink.

For detailed instructions, see the following topics: