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Do you mean you want to stop it from being sent or do you want to find a copy of the message?
In that case I am sorry to say but unless you are using a corporate Exchange email system or googles G-Mail system then you cannot recall an email message.
Basically when you write an email then hit send it immediately goes to your email server and is then passed on to the recipients email server and put into the recipients email inbox where it will wait for them to check their mail so about 10 - 30 seconds after you have sent it it will be either at your email server on its way out or sitting in the recipients email inbox.
I am truly sorry as I am sure this is not what you wanted to hear, however this is the way email works and there is nothing anyone can do to unsend the message or even retrieve it from the recipients end other than the person it was sent to.
Let us know if you need more information or clarification.
If the Outlook Express software is connected to a Microsoft Exchange corporate email server you can recall the email, however no matter if you are using an exchange client or not you only have 15 minutes to recall the email.
So has it been more than 15 minutes and are you using an exchange server not AOL like you asked about?