I've read numerous topics on this but they all suggest a server issue, which I think is unlikely becuase its only (just started) happening to 1 user.
SBS 2008 Domain (was upgraded from 2003 about 4 months ago)
Client is running XP with Outlook 2010 connected to Exchange 2007. Have 3rd party SSL.
Logs into the domain fine, and can access all domain features fine (printers, shared drives etc).
Symptoms are similar to when a network password expires but the client has not logged out, and Outlook wants a new password. However this machine has been doing it for 3 days (no prior problems, nothing has changed on the client or server in that time).
Rest of the domain PC's run a mix of XP and 7, all on Outlook 2010, only this one PC now has a problem.
Tried resetting the password. Tried recreating the Outlook profile, with and without the default HTTP Proxy settings. Rebooted numerous times. Checked IP and DNS settings.
Once credentials are put in, it works, more or less. Outlook says it is online. User has one incident when trying to send an email which got stuck in the Outbox, complaining the Exchange Server was unavilable.