Upon adding a second domain to an Exchange 2003 server, adding users/email addresses and restarting SMTP, incoming mail wouldn't come in.

The fix was to: 1. Install IIS6.0 Support Toolkit 2. Run 'Metabase Explorer' 3. Navigate to: LM-SmtpSvc-1-Domain 4. The new domain name was missing, so I copied/pasted an existing one