When deleted emails, contacts, or calendar entries are 'purged' from the Deleted Items folder, they go into a special area called Recoverable Items. Emails and other Outlook content stays in your Deleted Items folder for 90 days by default, but if you purge the Deleted Items folder, they go straight to recoverable items. After 90 days, items move to "recoverable items" and then stay there for 30 days.
You can refer to this University of Guelph page on How to Recover emails, or read a Microsoft article on the process.
If you cannot find an email, we recommend doing an advanced search in case the item was not deleted, but was instead moved to another folder by mistake.
If your account was previously compromised, there may be an Inbox Rule on your account that is changing the behaviour of your email. Here's how to check and remove Inbox Rules in the Outlook Web application and Inbox Rules in the Outlook Desktop client.