Possible causes include 1) an authoritative host wasn't found in DNS, 2) the smart host entry is incorrect, 3) the fully ...

{4}: Possible causes include 1) an authoritative host wasn't found in DNS, 2) the smart host entry is incorrect, 3) the fully qualified domain name (FQDN) in the Hosts file (this issue was fixed in Windows 2000 SP3), 4) a DNS failure or an invalid IP address for your smart host, 5) SMTP VS doesn't have a valid FQDN or SMTP VS FQDN lookup failed, and 6) a contact's SMTP domain doesn't resolve to any SMTP address spaces. Troubleshooting: Use Nslookup to check the DNS. Verify that the IP address is in IPv4 literal format. Verify that there is a valid DNS entry for the server or computer name in question.