How to forward email to external address in Exchange 2007

In the Exchange 2007 there is no problem if you want to forward email to another email address in the internal, use forward to, and choose the target internal mail address. (in mail flow settings tab, delivery options).
But if you want to forward to the external email address, you can't set it directly, you will not be able to write the external email address, the column is grey. you can only use browse menu, and choose internal mail address or group already listed.

forward to - cannot be set to external email address

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