During a search in Outlook you may come accross the following “We´re having trouble fetching results from the server. Some items may not be included in your search results.”

This issue is mostly a 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 a fixup attempt from a possible damaged mailbox. He then excluded the damaged elements. Then the Client side index runs out of sync and must be fixed.

Potential Solution #1

This issue can be seen if the Outlook client is missing the latest updates / service pack.
There have been bugs.

Potential solution #2

Try to rebuild the Outlook Search index. (See: How to Rebuild the Outlook Search Index)

Potential solution #3

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!

Potential solution #4

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.