Using Exchange 2003? You can extend the edge of your network as described here: http://support.microsoft.com/kb/823866#6. This reduces the need to use LDAP if you use something like MIMEDefang with its fabulous 'md_check_against_smtp_server' feature. First, create a recipient filter - To create a recipient filter, follow these steps: 1. Start Exchange System Manager. 2. Expand Global Settings, right-click Message Delivery, and then click Properties. 3. Click the Recipient Filtering tab. 4. [Intentionally Deleted by KAM] 5. To filter messages that are directed to e-mail addresses that do not exist in your organization, click to select the Filter recipients who are not in the directory check box. Next, apply the filters to the SMTP virtual server - Apply the connection filter or the recipient filter or both to the appropriate SMTP virtual servers You must enable the connection filters and the recipient filters on each SMTP virtual server where you want these settings to be applied. To apply a filter to a SMTP virtual server, follow these steps: 1. Start Exchange System Manager. 2. Expand Servers, expand Server Name, expand Protocols, and then expand SMTP. 3. Right-click the SMTP virtual server where you want to apply the filter, and then click Properties. 4. On the General tab, click Advanced. 5. Click the IP address that you want to apply the filter to, and then click Edit. 6. In the Identification dialog box, click to select either the Apply Connection Filter check box or the Apply Recipient Filter check box. 7. Click OK, click OK, click Apply, and then click OK. 8. Restart the SMTP virtual server where you applied the filter. 9. Repeat steps 2 through 8 for each virtual server where you want to apply the filter.