In both the online and desktop version of Outlook, recovering deleted e-mail is simple. With Office 365's Exchange Online, you can easily recover accidently deleted items.
It happens. You deleted an e-mail, emptied your Deleted Items folder, and then three days later you need that e-mail. Not to worry. When you delete an item permanently or empty your Deleted Items folder, those items go into the Recoverable Items folder.
To recover an item in Outlook Web App,
Right-click on the Deleted Items folder.
Click Recover Deleted Items.
Select the items you want to restore by selecting the boxes that appear when you hover over to the left of the e-mail.
Click the Recover Selected Items icon (yellow envelope with a blue arrow above the search box).
Use the search box to look for items in your Deleted Items folder but note that you cannot open an item until you recover it.
You can also purge items from the Recoverable Items folder but when you do so, you will not be able to recover those items again from the Recoverable Items folder.