Oh no, another Outlook password prompt issue.
This one has bit of a twist though. It's only happening on Windows XP clients - not on Windows 7 clients.
Also, the setup is slightly unorthodox.
The organization doesn't have any external connectibity apart of the Hub Transport Servers. Apart from the accepted domain, there isn't an external namespace.
I've had to generate a self signed certificate for the CASs and deploy this with a GPO. This works fine for Windows 7 clients - Outlook connects without an issue, but Windows XP clients prompt during startup. I've imported the certificate into the stores on the XP client and confirmed that IE doesn't have any certificate errors.
The CASs are load balanced with WNLB and RpcClientAccessServer set to the name of the CAS array on all databases.
The CAS Array has an A record of "casarray.doma" and a CNAME of "mail" and "autodiscover". The certificate contains mail and autodiscover. EWS/OAB/OWA internal URL is set to mail.domain and external URL is blank. I haven't set the internal URL for autodiscover.
Anyway, all works for Windows 7 but not for XP.
Any ideas where to being troubleshooting?