That error number means that Outlook was unable to find the Internet
address for the server name you entered - make sure you've entered the
server name correctly. That said, I and others have periodically hit this
problem here at Microsoft, and it appears to be an intermittent problem
with our corporate Domain Names Service (DNS), which is used to look up the
address of a server. You might ask your network provider to check that
their DNS is working properly.