Office 2003: http://office.microsoft.com/en-us/outlook-help/how-message-recall-works-HA001091760.aspx
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How to Unsend Email in Outlook 2007
http://www.ehow.com/how_8362269_unsend-email-outlook-2007.html
If you send a message to the wrong person by mistake, forget to include an attachment or notice an error in the content of an Outlook email, you may be able to use Outlook 2007’s recall facility to “unsend” the message if it hasn’t yet been read. This facility is only available if you use Outlook within a Microsoft Exchange Server environment.
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How to Unsend an Email in Outlook 
How to Recall a Sent Email in Outlook Instructions
- 3 Double-click on the mail to open it.
- 4 Click the “Message” tab on Outlook 2007’s ribbon menu, then click the “Other Actions” button.
- 5 Click “Recall This Message.”
- 6 Click the button next to “Delete unread copies of this message,” then click “OK.” If you wish, you can tick the “Tell me if recall succeeds or fails for each recipient” option. If you do, Outlook will report the success of the recall operation.
Tips & Warnings
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To increase your chance of success, try to recall your message as quickly as possible after sending it.
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If you want to replace the unread email with a new version, you can instead select the option of “Delete unread copies and replace with a new message.” Outlook then provides you with a new message window at the end of the process to create an alternative message.
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There is no guarantee of successful recall. The process relies on both you and your recipient using Outlook within an Exchange Server environment, and your recipient not having opened the email. You will not be able to recall messages sent to non-Exchange Server email accounts. Check the content of important emails before sending.
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If your recipient has mail forwarding set up to another account or a mobile device such as a smart phone, there is no guarantee the recalled mail will not have already arrived at the other destination.
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Recalling Email Messages in Outlook 2007
How many times have you sent out an email only to realize you forgot to include the attachment, or really shouldn’t have sent a response to the entire company? If you are using Outlook in an Exchange environment you can attempt to recall the message.
The best solution to this problem is to implement a delay before messages are sent out, but even in that scenario you still might let one slip through, so this is the second line of defense.
To recall the message, go to your Sent Items folder and then open the message you shouldn’t have sent.
On the Ribbon in the Actions group click on the “Other Actions” button and choose Recall This Message from the menu.
You’ll get a confirmation screen where you can decide to just delete the unread copies or replace with a new message. Since you are in a hurry the best bet is to just delete.
The critical checkbox below will let you find out if the recall succeeds or fails for each person you emailed. This way you can send a followup message to the people that already opened your first email, and maybe mitigate the damage a bit.
This doesn’t work flawlessly, but if you catch it in time you might be able to save some face.
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How do I unsend a message in Outlook?
In Outlook unsending a message is called Recall. Recall does not work in all situations.
There are several limitations of which you need to be aware.
Recall only works…
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for messages that are sent to other MCPS Outlook user’s
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for recipients using the Outlook client (not Outlook Web Access or OWA)
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if the recipient’s mailbox is open
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when the message is unread and in the recipient’s Inbox
To Recall a Message
1. Navigate to your Sent Items folder
2. Open the message you want to Recall
3. Select “Recall This Message…” from the Actions menu
4. To Recall the message select “Delete unread copies of this message”
5. To replace the message with a different message select “Delete unread copies and
replace with a new message”
6. To be notified as to whether or not your recall succeeds check the box next to “Tell me if
recall succeeds or fails for each recipient”
7. Click OK
NOTE:
Each recipient will get a message from you indicating that you wish to recall the message. Once
they open the Recall request, they are presented with a dialog box. If they have not read the original
message, they get receive a the dialog box below and the message and the Recall request are removed
from their Inbox:
If they have already read the original message, they get the dialog box below, the Recall request is
removed, but the original message stays:
Did the Recall Work?
If in Step 6 you checked the box to be notified about the success or failure of the recall, you will receive in
your account a notification from each recipient of the message indicating a success or failure.
Once you open these receipts, a permanent record is stamped on the original message. To view this
information, open the original message in your Sent Items folder, and click on the Tracking tab