Outlook prompting for password for SOME users since migration from Exchange 2003

I've had various "issues" along my migration path, and thanks to EE, these have all been sorted.

The scenario:-
Customer has recently been migrated from Exchange 2003 as part of SBS 2003 to Exchange 2010. All mailboxes, public folders etc have been migrated. Routing group connectors etc have been removed, and Exchange 2003 has now been uninstalled from SBS 2003.

All users are on Outlook 2010 (32-BIT) and SOME internal users are being prompted for credentials. Upon inspection, it would appear that RPC over HTTP has been enabled for all users. If I turn off this option within Outlook, then those people who WERE being prompted for credentials then work OK.  One user claimed that if she simply cancelled the prompt that she was still able to send and receive emails OK, whereas another user claims that he cannot.

Both the InternalURL and ExternalURL are set as https://exchange.mydomain.net/ews/exchange.asmx

The internal DNS correctly resolves to the internal URL for exchange.mydomain.net

AutoDiscoverServiceURI is set as https://exchange.mydomain.net/autodiscover/autodiscover.xml

What else could be the problem?
LVL 17
Chris MillardAsked:
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Will SzymkowskiSenior Solution ArchitectCommented:
I would be checking the Authentication you are using on your virtual directories. Also what authentication is configured within the Outlook client? I would set it to Negotiate if not already. Also, on an affected user do the following...
- open outlook
- hold ctrl+right click the Outlook icon in the system tray
- check the connection settings (to see what servers their Outlook is trying to connect to)
- also run the Test Email Auto Configuration (running the Autodiscover test, make sure that all of the virtual directories the client is using are correct).

Also check the applicaiton log on the Clients machine as well.

Will.

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Chris MillardAuthor Commented:
I've not been in a position to check the above yet, but I contacted the customer for an update and have been informed that no one has (so far) reported this problem since the beginning of last week.
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