If the mail is no longer in the Deleted Items folder or in the recovery folder linked at the top of the mailbox in Outlook on the web, you can try an Office addin called Email Recovery. It can recover mail that is out of the recovery folder and waiting for Exchange to purge it - but the time frame for recovery is short.
https://appsource.microsoft.com/product/office/WA104380447
You want to choose Purges - it will create a new folder and put all of the recovered mail into it. After you are done, delete the folder then delete it from the deleted folder.
