Hi Diana,
Based on the current situation, I would like to collect some information.
1. Can you enter your mailbox successfully in OWA?
2. Have you tried to configure your mailbox in Outlook of updated version, such Outlook 2010 or 2013?
3. Could you provide me with the screenshot about "it comes back with a red cross but no error message"?
4. If you has installed Office 2007 SP3 updates.
In addition, you could try to start Outlook 2007 in safe mode and check if the issue persists.
To start Outlook in safe mode, use the /Safe command-line switch. To determine whether you are running in safe mode, view the title bar of Outlook.
Thanks,
Kid Yang