When an Office 365 user searches their mailbox in Outlook on the web (formerly known as Outlook Web App), they notice that a large number of messages is missing from the search results. Additionally, gaps may be present in the date range of items that are returned in the search results.
This occurs if the search returns more than 250 items.
Do any of the following:
Reduce the scope to search only specific folders or subfolders.
Use more specific search terms.
Search the mailbox by using Microsoft Outlook.
Outlook on the web displays up to 250 items in the search results. Be aware that to reduce the time to show search results, users may only get a subset of the results. In this scenario, click Get more results at the bottom of the search results to view additional results up to the limit of 250 items.