If you want to recover a deleted email in Outlook, you have a couple of options depending on where you are in the deletion process.

If you just deleted the email and you want to recover it, you can press Ctrl + Z to undo the delete.

If the deletion of the email did not just occur, it may still be in Deleted Items folder. Just navigate to the Deleted Items folder, find the email you want to recover, and drag it to one of your other folders--you can drag the deleted email to any folder including the Inbox.

If the email was deleted so long ago that it has since been deleted from the Deleted Items folder, then you need to use a third-party tool to recover the message. GetData makes a product called "Recover My Email" that allows you to do just that. While it is not a free program, it does allow you to install it and see if it can recover your email messages of interest before you decide to purchase. So at least you know ahead of time if spending your money will result in recovering your deleted email.