Office 365 (Outlook for iOS) - Search Messages
This document explains how to search through your messages with a keyword or phrase
At the top of the screen, tap the Search icon.
Enter the word or phrase that you would like to search through your emails for. All emails containing your entry will be displayed.
For information on how to narrow your search criteria for better searches, click the link to the following Microsoft Support page: Learn to Narrow Your Search Criteria.