Exchange does not natively reject invalid addresses during the SMTP connection. This can cause problems for MailCleaner customers who select the SMTP Callout option.
However, Exchange 2003 can be configured to reject invalid addresses, resulting in a 550 5.5.1 User unknown error. MailCleaner, in turn, can bounce these messages and your user count will reflect only valid addresses in your system. To configure this ability on the Exchange server, follow the directions below.
NOTE: These instructions apply to Exchange 2003 only; neither Exchange 2000 nor 5.5 support this option.
Enable directory lookup for recipients in the recipient filter
1. Open Exchange System Manager.
2. Open Global Settings, right-click on Message Delivery, choose Properties
3. Choose the "Recipient Filtering" tab
4. Check the box "Filter recipients who are not in the Directory"
5. Click OK to close.
Enable the recipient filter on the SMTP protocol binding that accepts mail from the Internet
1. Navigate to the SMTP Virtual Server that listens on the Internet (repeat all of these steps if you have more than one)
2. Right-click on the SMTP Virtual Server, choose Properties
3. On the "General" tab (already open), click the "Advanced..." button next to IP address
4. Choose the IP/port binding that corresponds to the one that listens on the Internet. Either double-click or click the "Edit..." button.
5. Click the check box next to "Apply Recipient Filter"
6. Click OK three times to close this.
Now, when someone does a RCPT TO: invaliduser@yourdomain, they will get a:
550 5.5.1 User unknown
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