After you delete an email in Office 365, it is automatically moved to the Deleted Items folder. It will remain in there until the Deleted Items folder is emptied, either by using the empty deleted items option or the emptying the deleted items folder when I sign out option.
However, if you empty the Deleted Items folder and then discover that you had deleted an email by mistake, it is possible to use the Recover Deleted Items option to restore it.
Manually emptying the deleted items folder
1. In Office 365 Email locate the Deleted Items folder
2. Right click the Deleted Items folder and select Empty folder
3. You will be asked to confirm you want to do this. Select ok and the items will be deleted
Automatically emptying the deleted items folder when you sign out
1. In Office 365 Email select the settings cog
2. Then, under the Your app settings, select Mail
3. Under Automatic processing, choose Message options
4. Check the box next to Empty the Deleted Items folder when I sign out
5. Save
Recovering deleted items
Microsoft's instructions for recovering deleted items in Office 365 Email