Open up the Exchange Management Console in 2007.  Navigate to the system drive %Program FilesMicrosoftExchange ServerScripts  then run the following commands\n./install-AntispamAgents.ps1\n
Restart-Service MSExchangeTransport
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Once this is done if you are using the server as a hub transport and it is the only Exchange server you have make sure to add it to the internal servers list.  This list can be found in the Exchange Management Console under Organization Configuration>Hub Transport Global settings tab>Transport Settings>Message Delivery tab.
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More information can be found here:  http://technet.microsoft.com/en-us/library/bb201691.aspx