users are complaining that they receive this message regularly.
Some of the users have 6.5Gb, 4.5Gb 2.5Gb sized mail stores.
Microsoft says
This issue occurs when an Outlook user works with items in a folder that contains many items. Outlook must perform several operations against the Exchange server to retrieve the contents of a folder. Therefore, when there are many items in a folder, additional processing is required to respond to the Outlook requests.
Andy Says:
you should enforce "Outlook Cached Mode" to your users for their own good.
Here are some good research tips on this issue
http://anandpv.spaces.live.com/Blog/cns!AFCCA5892B178862!630.entry
http://anandpv.spaces.live.com/blog/cns!AFCCA5892B178862!657.entry
http://support.microsoft.com/kb/905803