Using Outlook for iOS in business environments for email and scheduling
If you're using Outlook, first check your Deleted Items, Archive, and Junk Email folders since older messages may have been moved there.
If you're searching from the Outlook search bar, try searching by the sender's email address or a word from the subject instead of only changing the date range. Also make sure the search scope is set to All Mailboxes or All Outlook Items, depending on your Outlook version.
If you still can't find them, check Outlook on the web and search for the same message there. If the emails are also missing online, they may have been deleted or moved by an account retention/archive policy.
It would help to know whether you're using New Outlook, Classic Outlook, or Outlook.com, and whether other emails from around that time are still available.