Changes to the email server as with any server are often planned and usually involve patches, fixes and upgrades. Other changes are related to configuration settings and performance tuning. Sometimes the change might be to allow more email traffic to be handled by the existing email server.

One system administrator related that a change was made to the server that would allow more than just receiving POP3 email. Then the server was configured as the primary MX server in the global DNS settings. The result was that both incoming and outgoing email messages were held in the “Messages awaiting directory lookup” queue of Exchange Server 2003. This queue is also known as the pre-categorization queue.

How the process works is that messages from the pre-submission queue are placed into the pre-categorization queue by the advanced queuing engine. Once the messages are placed into the pre-categorization queue then the categorizer can process those messages.

The categorizer is responsible for resolving message addresses from the sender and receiver fields using Active Directory. Any distribution lists will be expanded as necessary. The categorizer also validates any restrictions or limits as related to the sender and receiver addresses.

Messages that have not yet been resolved are held in the Messages Awaiting Directory Lookup queue. Sometimes those unresolved messages can create a backup in the Messages Awaiting Directory Lookup queue if those messages are not resolved in a timely manner. The causes for the backup can range from slow services such as the global catalog lookup to inaccessible catalog servers. Messages can backup the queue on front end servers if the Exchange Information Store service has been disabled for some reason. It is best not to disable this functionality on the front end servers.

Diagnostic information can be collected by an administrator by increasing the diagnostic logging for the MSExchangeDSAccess and MSExchangeTransport services. An administrator can increase the logging by performing the following steps:

1. Click Start, then Run and type regedit in the command field. Hit ENTER.
2. Locate and then click the following registry subkey: HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesMSExchangeTransportDiagnostics
3. Set all the categories under this registry subkey to the value of “5”.
4. Locate and then click the following registry subkey:
   HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesMSExchangeDSAccessDiagnostics
5. Set all categories under this registry subkey to the value of “5”.
6. Exit the Registry Editor.

The following events may be logged in the Application log after you increase the diagnostic logging for the MSExchangeDSAccess and MSExchangeTransport services:

Event ID: 8024
Source:ExchangeAL
Category: LDAP Operations
Description: LDAP Search Initial Page on directory DirectoryName at base ‘DistinguishedName’ was unsuccessful. Directory returned the LDAP error:[0x34] Unavailable

Event ID: 9004
Source: MSExchangeTransport
Category: Categorizer
Type: Error
Description: Categorizer encountered a hard error while processing a message. While processing User ‘smtp:user@example.com’, the function ‘CPhatCat::HrIsRecipientInSameRG’ called ‘MDAGetInfo’ which returned error code ’0×80040920′ (). A DSN has been generated.

Event ID: 9003
Source: MSExchangeTransport
Category: Categorizer
Type: Warning
Description: Categorizer is temporarily unable to process a message. While processing user ‘smtp:user@domain.com’, the function ‘CPhatCat::CompleteUserMailDrop’ called ‘SetTargetServer’ which returned error code ’0xc0040559′ (The categorizer encountered an error. The operation will be restarted.).

Event ID: 929
Source: MSExchangeTransport
Category: Routing Engine/Service
Type: Error
Description: Failed in reading Connector’s DS Info Process Id: 8176 Process location: C:WINDOWSsystem32inetsrvinetinfo.exe ConnectorDN: DistinguishedName Hr:c004038a Attribute:[msExchSourceBridgeheadServersDN]

Event ID: 6004
Source: MSExchangeTransport
Category: Categorizer
Type: Warning
Description: The categorizer is unable to categorize messages due to a retryable error.

Event ID: 9035
Source: MSExchangeTransport
Category: Categorizer
Type: Warning
Description: Categorization of the message failed with a retryable error. Either some of the admin objects were renamed recently and changes were not picked or journaling was turned on for MDB (DistinguishedName) and the journal recipient’s mailbox is deleted.

If these Event IDs are logged then an administrator can use any of the following methods to resolve this issue:

1. Verify that the global catalog server is available.
2. Verify that the Exchange servers have sufficient permissions.
3. Verify that the attributes of user object are correct.
4. Verify that the message archive location is valid if message archiving is enabled.
5. Verify that the mailbox store on the SMTP bridgehead server is mounted.
6. Verify whether this issue occurs when you send messages to a large distribution list.
7. Verify whether any third-party software is installed on the Exchange server.