Preview will not default mail to Entourage

Our default email system is Entourage. But if we have a pdf open in Preview and click 'file, mail selected pdf document' it opens Mail and asks us to set up an account instead of defaulting to Entourage. With Word, Excel, etc 'file, send to' automatically opens Entourage and attaches to emails OK.

How do we get Preview to accept Entourage as the default mail system?