XCON: How to Manually Flush Message Transfer Agent Distribution List Cache

If you send a message to a distribution list (DL) from which a mailbox has been deleted, you may receive a non-delivery report (NDR).

This issue can occur because when you send a message to a DL, the message transfer agent (MTA) copies the DL to a cache. When the DL is modified, the updated DL is copied to the cache. However, when you delete a mailbox that is a member of a DL, the MTA does not indicate that the DL has been modified. Therefore, the cache is not updated. A message that is sent to the DL is sent to the deleted mailbox and a non-delivery report (NDR) is generated.

Note that DLs are not cached in Exchange Server 4.0. Therefore, this issue does not occur in Exchange Server 4.0.

To work around this issue, use one of the two following methods to manually flush the MTA DL cache:

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Stop and restart the MTA. You may not want to use this method in situations where there are a large number of messages in the MTA.

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Manually flush the MTA DL cache by using the Exchange Server Administrator program: 1. Start the Administrator program. 2. Click the DL that contained the recipient that was deleted, and then open the DL properties by using one of the following methods: � Double-click the DL. � Click the DL, and then click Properties on the File menu. 3. Click the General tab, and then click Modify. 4. Click OK without making changes. 5. Click Apply to apply the changes to the DL properties. When the Last Modified Date changes, the cache has been flushed. 6. Click OK to close the DL properties.

Each DL has a When-Changed attribute that indicates when the DL was most recently modified. When the MTA expands a DL, the MTA checks this attribute to determine if the DL has been modified. If the DL has been modified, the MTA updates the cache. Note that if the DL is modified on a different server, this attribute is not set until the directory is replicated.

When an object is deleted from the directory, all of the object's links and backlinks are removed. However, when an object is deleted, the time stamp on objects that were linked to the deleted object is not affected. For example, if a mailbox is a member of a DL and the mailbox is deleted, the time stamp on the DL is not updated. Furthermore, when you delete a DL, the time stamp on the associated mailboxes is not updated. This behavior occurs in the directory services that are included with Exchange Server 4.0, 5.0, 5.5, and Microsoft Windows 2000.

The directory service does not update the time stamp on objects that are linked to a deleted object, and Microsoft does not plan to modify the directory service to do so. In some cases, the DL and the deleted mailbox are located in different sites, and in these cases the DL cannot be updated when a mailbox is deleted because the directory service only contains a writable copy of the objects from one particular site. Note that links are only removed on the local server; these changes are not replicated.

Links are not like other attributes that only apply to one particular object. Instead, links apply to multiple objects and can be modified when either of the associated objects are modified. However, when a link is modified, the time stamp on both objects is not updated. For example, when you add a mailbox to a DL, the time stamp of the DL is modified and additional links are created, but the mailbox's time stamp is not modified (even though the mailbox's Is-Member-Of attribute is modified). Similarly, when you delete a mailbox that is a member of a DL, the mailbox's time stamp is modified and all the associated links are removed, but the time stamp of the DL is not modified.

Therefore, when you remove a mailbox from a DL, the time stamp of the DL is modified, which causes the MTA to behave differently than the MTA behaves when you delete the mailbox without removing the mailbox from the DL.

When you delete a mailbox that is a member of a DL, the When-Changed attribute of the DL is not updated. Therefore, if the DL has already been cached, the cache is not updated because the MTA does not determine that the DL has been modified. If you send a message to the DL, the message is sent to the deleted mailbox and you receive an NDR.