Find your (soon to be recalled) email in the "Sent Items" folder in Outlook 2007, double-click on it in order to open it.

In the "Actions" section of the Ribbon click "Other Actions", select "Recall This Message"

Decide how you would like it to be recalled, either checking "Delete unread copies" or "Delete unread copies and replace with a new message"
If successful you should receive a successful confirmation email.