If you sent a message to someone by mistake or you forgot to add or attach something you can try recalling the message.
Outlook Message Recall feature only works if both sender and recipient are using Outlook and their mailboxes are hosted on Microsoft Exchange Server.

Recall a message in MS Outlook 2003

Mailbox > Sent Items > [your message] (double click to open) > Actions > Recall this message

Recall a message in MS Outlook 2007

Mailbox > Sent Items > [your message] (double click to open) > Other Actions > Recall this Message

Recall a message in MS Outlook 2010

Mailbox  > Sent Items > [your message] (double click to open) > Actions > Recall This Message

You will be informed by email whether recall succeeded or failed.

Recall will succeed if recipient has not read the message yet.
Recall will fail if message has been read, moved from the Inbox to another folder, or if recipient is not using MS Outlook.