Any limit on the number of messages you can send is determined by your mail provider not Outlook. But with a list of 6,000 addresses, I'd look into a purpose-built mailing list server, either hosted or in-house, that can automatically handle the bounces and unsubscribes for you.

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Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
http://www.turtleflock.com/olconfig/index.htm
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers
http://www.outlookcode.com/jumpstart.aspx
> Hi there, over the past few years of trading we have built up a list of 6000
> email address from our customers. I would like to send a newsletter to our
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>
> Amit