在Web里好像没办法做，要到Outlook里去才行

Recall a message

In Mail, in the Navigation Pane, click Sent Items.
Open the message that you want to recall.

On the Message tab, in the Actions group, click Other Actions, and then click Recall This Message.

Click Delete unread copies of this message.

Note If you are sending the message to a large number of people, you may want to clear the Tell me if recall succeeds or fails for each recipient check box.