Consider the following scenario:
You configure a federation trust between two organizations by using Microsoft Federation Gateway.
The first organization is a mixed Exchange Server 2003 and Exchange Server 2010 environment.
The second organization is an Exchange Server 2010 only environment.
A user in the second organization looks up the free/busy information of an Exchange Server 2003 mailbox user in the first organization.
In this scenario, hash marks are displayed instead of the Free/Busy information.
When Exchange 2010 in the remote organization detects an autodiscover query being performed for a legacy Exchange 2003 mailbox, Exchange 2010 will fail the lookup because Exchange 2003 does not support Autodiscover.
To resolve this issue, install the following update rollup:
2579150 Description of Update Rollup 4 for Exchange Server 2010 Service Pack 1
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
A separate issue with Exchange 2003 Cross-Org lookups has been identified due to MergedOnly Free/Busy information being returned. For more information , click the following article number to view the article in the Microsoft Knowledge Base:
2567863 Free/Busy information may not be returned when a Cross-Org query is performed from a legacy Exchange user