XCON: Distribution List (DL) Expansion

Distribution List (DL) expansion can place a heavy burden on the Message Transfer Agent (MTA) service. In organizations that have large DLs, it may be necessary to control which server is responsible for DL expansion. This article explains the settings that can be changed in the Exchange Server Administrator program to control DL expansion.

The default for this feature is for any server to be able to expand any DL homed in the local site.

DL Expansion is controlled by the following two settings:

• On the General Property tab of the Message Transfer Agent, there is a checkbox to Expand remote distribution lists locally.
  
  This indicates that DLs, which are homed in a remote site, will be expanded locally.
• The Expansion Server option on the properties of the DL.
  
  Setting the Expansion Server to a server within the DL's home site indicates that only the MTA on the chosen server does the expansion regardless of all other parameters.

Sample Scenario:

Site1 has a site connector to Site2 where a DL resides. Directory replication is configured between the two sites. All MTAs in Site1 are set to not expand remote distribution lists locally. The DL does not have an expansion server set.

A message generated in Site 1 to the DL forwards the message to the bridgehead server in the DL's home site. The bridgehead server in Site2 expands the DL, and then fans out messages to recipients on the DL.

Because the sites are joined via a site connector, they can be configured to use more than one server in a site as the target. The server that expands and fans out the message may vary from message to message. However, if sites are connected via an X.400 connector, Dynamic RAS, or Internet Mail Service, the targeted server is one single server and will be responsible for expanding the DL.