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If you use Outlook Desktop for Windows you may find that when searching for an email, search results do not sort chronologically. Consequently you are seeing older emails instead of the email you seek.
To force Outlook to list email search results by chronology, do the following:
In the upper right, click the gear icon. In the resulting dialog window, on the lefthand side choose General. Then, choose Search from the General menu options.
On the Search settings page, under Search results you see "Show most relevant search results at the top." Uncheck this box. Outlook will no longer try to guess which email is most relevant to your search, and instead will revert to listing (reverse) chronologically, where the latest emails are a the top of results.