t’s common to install the SMTP service on servers that require limited mail-sending capabilities–for example, on a Microsoft SharePoint server. I recently had a problem in which the box got a new IP address, and at that point the services that used the SMTP service could no longer send email. The problem was that the server had a relay restriction list in place, and the list didn’t have the new IP address listed.
To resolve the problem, you need to update the relay list. Start the Microsoft Management Console (MMC) Internet Information Services Manager snap-in by accessing Start, Programs, Administrative Tools, Internet Information Services (IIS) Manager. Right-click the SMTP virtual server and select Properties, then select the Access tab. Click the Relay button. Be sure to add the IP address of the server trying to connect or a subnet containing the servers trying to send via the server by clicking the Add button. When you’re finished, click OK on all dialog boxes.