Current Status: Issue – no issue
A few customers, partners, and individual folks in the Exchange community have reported that a few of the Exchange 2010 users running Outlook 2003 randomly have issues connecting to their mailbox. After examining the mail profile in Outlook 2003, you can see the Exchange server name appears as Instance-<GUID> where <GUID> is a randomly generated GUID (or number). Re-resolving (‘Check Name’) the appropriate server name in the Outlook mail profile resolves the issue.
Figure 11: Instance-GUID issue in Outlook 2003
Seeing Instance-<GUID> in conversation between an Outlook 2003 client and an Exchange 2010 server is normal (when Instance-<GUID> appears inside Outlook in the folders list view on left hand side); It does NOT mean you are hitting this particular issue. You experience this issue if the Outlook mail profile gets updated to refer to the Exchange server as Instance-GUID.
The problem behavior can be triggered by networking issues, patch applications, server reboots, etc. It requires Outlook 2003 to lose connectivity to Exchange and have a delegate mailbox logon attempted on a connection before the primary mailbox logon is attempted on that connection. This can affect users who are accessing shared mailboxes or folders of other users from their Outlook 2003 while connecting to Exchange 2010.
Currently there is no proper resolution available for this issue. However, to remediate this issue temporarily, simply re-resolve the Exchange server name in the Outlook mail profile. If possible, you may also remove the shared mailboxes or shared folders (like calendars) from the profile, preventing this issue from occurring in the future or create a new mail profile in Outlook.
Microsoft has confirmed this to be a bug and the bug was assigned to the Outlook team within Microsoft. It has been fixed and a hotfix can be downloaded here: http://support.microsoft.com/kb/2510153