When you expand a distribution list in Microsoft Outlook 2010, the names of the members of the distribution list are not displayed in the Members box.
This issue occurs if the distribution list contains more than 5,000 members. The maximum number of members that a distribution list can contain in Outlook 2010 is 5,000.
To resolve this issue, apply the following update:
2598318 Description of the Outlook 2010 hotfix package (Outlook-x-none.msp): April 24, 2012
Note For this fix to work, the Exchange administrator must adjust a server-side time-out policy. The server-side throttling policy causes a random failure because the expansion takes too much time to access Active Directory Domain Services (AD DS). To avoid this failure, the administrator must increase the RCAPercentTimeInAD parameter to a larger value by using the Set-ThrottlingPolicy cmdlet.
For more information about this cmdlet, go to the following Microsoft website:
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.