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Recently, I discovered a problem that occurs when you delete a mailbox when the recipient is a member of an intersite distribution list (DL): Exchange Server continues to deliver mail to that recipient's mail server when someone sends mail to that DL. In addition, even after Exchange replicates the change to all servers in the organization, the sender receives a nondelivery report (NDR) for the deleted recipient. Although the Global Address List (GAL) DL properties show that the recipient is no longer a member of that DL, Exchange hasn't updated the Message Transfer Agent (MTA) DL cache with DL deletions and therefore forwards mail to the deleted recipient's old mail server. Exchange doesn't purge the MTA DL cache automatically even though you're updating the list with the new information because the directory doesn't update the DL's when-changed attribute when you delete the mailbox and thus implicitly remove it from the DL.

This behavior is similar to the behavior of the LMHOSTS file and the NetBIOS name cache. Although you make new LMHOSTS name entries in the file, you must purge the NetBIOS name cache before the new LMHOSTS entries appear and you can perform name resolution for those new host names.

You can obtain a fix for the problem from Microsoft Product Support Services (PSS); however, Microsoft recommends that you adopt the fix only if you're severely affected by the problem. The Microsoft articles "XCON: MTA Returns NDR for Deleted Mailbox when Message Sent to DL" (http://support.microsoft.com/support/kb/ articles/q194/5/04.asp) and "XCON: Intersite Message Transfer Agent Distribution List Cache Issue" (http://support.microsoft.com/support/kb/ articles/q229/7/28.asp) discuss the problem.

Microsoft's workaround is to recycle the MTA on the Exchange server where you created the DL. However, because I didn't want to stop and start the MTA service, I developed another workaround. On the Exchange server on which you created the DL, follow these steps:

  1. Start Microsoft Exchange Administrator.
  2. Select the problem DL, and open its Properties page.
  3. On the General tab, click Modify.
  4. Without making any changes to the list, click OK.
  5. Back on the DL's Properties page, click Apply. Notice that the Last Modified Date in the lower right corner has changed.
  6. Click OK to close the Properties page.

These steps automatically purge the MTA DL cache and update it with the latest information because you're fooling the cache into believing that you've modified the cache instead of deleted a mailbox. The change is instantaneous, and you don't have to stop your Exchange server to accomplish it.