There are a number of tools you can use (or VBA) that will walk though
messages and create contacts from existing email addresses.
http://www.slipstick.com/addins/contacts_entry.asp

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Diane Poremsky [MVP - Outlook]
Author, Teach Yourself Outlook 2003 in 24 Hours
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> I have received thousands of emails from clients of mine and I would like
> to
> build a mailing list to them can I import their email addresses into a
> contacts folder.
You need to look for third party software that can do this. Outlook has no
such function.
Start here:
http://www.slipstick.com/addins/contacts_entry.asp

Signature
Russ Valentine
[MVP-Outlook]
>I have received thousands of emails from clients of mine and I would like
>to
> build a mailing list to them can I import their email addresses into a
> contacts folder.