I had this come up with a client and promised Matt I would add the solution to the forum.
The client purchased a new computer and couldn't send e-mail form Outlook.The problem was not Outlook or the server, but the client's firewall. It was blocking Outlook every time a message was sent. The error message was "no response from server". The client disabled the Anti-Virus program and the associated fire wall. The e-mail worked perfectly. The AV program and firewall was enabled and a new rule established (this often happens automatically) and the problem was solved. Hope this helps.