For Microsoft Outlook 2003:

1. Go to the Sent Items folder.

2. Find the message you want recalled and double-click it.

3. Go to the Actions menu and select Recall This Message.

4. To recall the message:

Select Delete unread copies of this message.
(Note: the recipient needs to have Outlook opened for the message to be deleted)

To replace the message:

Select Delete unread copies and replace with a new message, click OK, and type your new message.

To be notified about the success of the recall or replacement:

Check the Tell me if recall succeeds or fails for each recipient check box.

5. Click OK.

UPDATE: How To Recall a Sent Message in Microsoft Outlook 2007:

1. Click on Sent Items.

2. Find the message you want recalled and double-click it to open.

3. Go to the Ribbon.

4. In the Actions section, click Other Actions and select Recall This Message.

5. Select Delete unread copies of this message.

6. To be notified about the success of the recall, check the Tell me if recall succeeds or fails for each recipient checkbox.

7. Click OK.

Related Links:

Hosted Apps – Exchange SharePoint

Business Email Hosting

Hosted SharePoint Exchange Services

Creating Contacts in Hosted Exchange

Hosted Exchange Plans