In Exchange Server 2003, Recipient Filtering has an option to "Filter recipients who are not in the directory". In Exchange Server 2007, this is called Recipient Validation. In a topology with Edge Transport server(s) deployed, Hub Transport servers transmit a list of valid SMTP addresses to the Edge. This is not required in environments without Edge servers - HT servers can look-up AD directly and drop messages for non-existent recipients.
The Exchange Management Console (in Exchange 2007 RTM) does not have the option to turn on Recipient Validation. Use the following command to turn it on:
Set-RecipientFilterConfig -RecipientValidationEnabled:$trueTo check your Recipient Filtering configuration:
Get-RecipientFilterConfig | fl
In the above example, results from the Get-RecipientFilteringConfig cmdlet are piped to the Format-List cmdlet. fl is an alias for Format-List. See Using the Exchange Management Shell for some helpful information on Exchange shell.