unsend message

In Outlook 97 you can open the sent message, go under tools and "Recall this Message"

In newer versions, the option to Recall is under the Actions menu (when you have the sent message open).

It will inform you whether it is successful or not (but of course will not help if it has been delivered/read).

Yes it can be done if you are using Corporate/Workgroup. Here's the documentation:
(Type the Q number in the search box at support.microsoft.com)

How to Use the Recall Message Feature

Q197094

"To recall or replace a message, follow these steps:
On View menu, click Folder List.
On the Folder list, click Sent Items.
Open the message you want to recall or replace.
On the Actions menu, click Recall This Message.
Click to select, "Delete unread copies of this message", to recall the message. To replace the message, click to select "Delete unread copies and replace with a new message", click OK, and then type your new message and send it.
To receive notification about the success of the recall or replacement, click to select"Tell me if recall succeeds or fails for each recipient" check box.

Click OK."