Outlook 2007 + Exchange 2007 provide an option to send OOA responses only to people in your Contacts folder.

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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
> when using exchange "out of office assistant" outlook must permit me to
> choose who will recieve the autoreply message. This will prevent spammers
[quoted text clipped - 8 lines]
>
> http://www.microsoft.com/office/community/en-us/default.mspx?mid=1aae2790-b41d-4
6c5-9998-466a3b925d95&dg=microsoft.public.outlook.installation
> when using exchange "out of office assistant" outlook must permit me
> to choose who will recieve the autoreply message. This will prevent
> spammers from knowing my account is active
Spammers don't monitor the replies they get back. Besides, by default, the
OOA doesn't respond to Internet addresses.

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Brian Tillman