source: http://www.securityfocus.com/bid/2823/info

Outlook Express is the standard e-mail client that is shipped with Microsoft Windows 9x/ME/NT.

The address book in Outlook Express is normally configured to make entries for all addresses that are replied to by the user of the mail client. An attacker may construct a message header that tricks Address Book into making an entry for an untrusted user under the guise of a trusted one. This is done by sending a message with a misleading "From:" field. When the message is replied to then Address Book will make an entry which actually replies to the attacker. 

Situation: 2 good users Target1 and Target2 with addresses [email protected] and
[email protected] and one bad user Attacker, [email protected]. Imagine Attacker wants to get
messages Target1 sends to Target2. Scenario:

1. Attacker composes message with headers:

From: "[email protected]" <[email protected]>
Reply-To: "[email protected]" <[email protected]>
To: Target1 <[email protected]>
Subject: how to catch you on Friday?

and sends it to [email protected]

2. Target1 receives mail, which looks absolutely like mail received from
[email protected] and replies it. Reply will be received by Attacker. In this case
new entry is created in address book pointing NAME "[email protected]" to
ADDRESS [email protected].

3. Now, if while composing new message Target1 directly types e-mail
address [email protected] instead of Target2, Outlook will compose address as
"[email protected]" <[email protected]> and message will be received by Attacker.