Outlook Web Access gives you plenty of opportunity to change your mind after deleting an e-mail message. You can retrieve it from your Deleted Items folder, since it remains there until you empty that. Even if you empty Deleted Items, it is still possible to retrieve messages for a couple of weeks.
This has advantages and disadvantages
Deleted Items Folder
The Deleted Items folder shows up on your list of folders. Whenever you delete a message, it is automatically moved to there. If you accidentally deleted a message, click on "Deleted Items" in Outlook Web Access. Find the message you want and drag it to your Inbox.
To empty the folder:
Recover (and permanently delete) Deleted Items
The recovery information is on the "Options" bar at the lower left of the screen.
Click on the items you want to recover or delete, then click on either "Recover" or "Permanently Delete." "Recover" will move items back into the "Deleted Items" folder, where you can drag and drop them to your Inbox. "Permanently Delete" will remove the items completely for the system -- useful if you're over quota.