This document describes how to recall sent emails in Outlook 2003:

To recall a sent email, do the following:

1. In Mail, in the Navigation Pane, click Sent Items.

2. Open the message you want to recall or replace.

3. On the Actions menu, click Recall This Message.

4. Click Delete unread copies of this message. 

This is very useful when you have sent a mail too soon (before completion for instance) or with a wrong file in attachment. I tried this function this week at work. I noticed that it works only if the recipient is a person (it doesn’t work for mail group).