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This issue is mostly an client side issue and could happen if the Outlook folders show a number of items that is less than expected and older items may appear to be missing. Such an issues could happen if some elements where deleted by an administrator due to an fixup attempt from an possible damaged mailbox. He then excluded the damaged elements. Then the Client side index runs out of sync and must be fixed.
Possible solution 01:
This issue can also be seen if the Outlook client is missing the latest updates / service pack. There was a bug in the indexer component which caused the problem in the past.
Possible solution 02:
Try to rebuild the Outlook Search index. (See: How to Rebuild the Outlook Search Index)
Possible solution 03:
Check if you have set the value to All under the Use Cached Exchange Mode section. If no, set it follow the steps below:
a. Click File->Account Settings->Account Settings.
b. Select your account and then click Change.
c. Change the value to All and then click Next->Finish.
Note: If you do not see that option it might be on purpose and hidden by your administrator!
Possible solution 04:
Most often, the reason for this error message is that the Windows or Outlook profile is damaged. So try to rebuild/reset the Outlook profile.
Possible solution 05:
It might be that the Exchange Administrator disabled full text indexing (e.g. stopped the needed service on the Exchange Server) or disabled that via:
Set-MailboxDatabase "Mailbox Database (EXCH01)" -IndexEnabled $false
To enable that again use (as mentioned here):
Set-MailboxDatabase "Mailbox Database (EXCH01)" -IndexEnabled $true