This condition could also be caused by reinstalling the Internet Information Services (IIS) SMTP service on an Exchange 2000 server. As described in Microsoft Knowledge Base article 290290, the problem occurs because, although SMTP has been restored, the Exchange extensions to SMTP have not been. If you recently reinstalled IIS or the IIS SMTP service on this system, then this article can help you fix the problem.
If you haven't recently reinstalled the SMTP service or IIS, then it could be a simple DNS issue (e.g., incorrect DNS settings, typo in the domain name, missing DNS records, etc.). In that case, I recommend checking your TCP/IP settings and DNS settings, and using NSLOOKUP on the Exchange server to verify MX records. Microsoft Knowledge Base article 203204 explains how to do this.
If this is a new SMTP Connector, you might also want to double check its configuration. Microsoft Knowledge Base article 319426 describes in detail how to do this.
This was first published in October 2003