Duplicate messages are sent to an external recipient if the recipient is included in multiple distribution lists in an Exchange Server 2007 environment

Consider the following scenario. An Exchange 2007 user sends a message to a parent distribution group that includes child distribution groups. More than one child distribution group includes the same one-off recipient. In this scenario, the message is delivered multiple times to the target system of the one-off recipient. If the target system does not detect the message duplication, the recipient receives multiple copies of the same message.

Note A one-off recipient is a recipient whose address is not in the Active Directory.

This problem occurs because the Exchange Server 2007 server uses the Exchange store to detect duplicates. However, this method does not work for one-off recipients.

To resolve this problem, install Update 8 for Exchange 2007 Service Pack 1.

For more information about Update Rollup 8 for Exchange Server 2007 Service Pack 1, see the following Exchange Help topic: For more information about how to obtain the latest Exchange service pack or update rollup, see the following Exchange Help topic:After you apply this fix, Exchange 2007 keeps a configurable 100 KB * 16 byte cache for contact items and for distribution groups when you expand distribution groups to reduce duplication.

Note You can configure the cache size to meet your actual needs. To change the default cache size manually, follow these steps:

1. Stop the Exchange Transport service on the Transport server.
2. Locate the EdgeTransport.exe.config file. This file is located in the following folder:
   <drive> :\Program Files\Microsoft\Exchange Server\Bin
3. Add the following entry between the <appSettings> element and the </appSettings> element of the EdgeTransport.exe.config file:

   <add key="MaxResolveRecipientCacheSize" value="xxxx" /> 

4. Restart the Transport service.

Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.