How to make the field E-Mail in Contact Form 7 optional?

At the form (Contact Form 7) the email should be optional.
That for I removed the “*” from 
[email* your-email autocomplete:email]
But then I get a error at the Mail tab. 

Is it possible to use email optional, but still use the reply-to?
May be is it possible to set a default value on your-email an than there is no error at the mail-tab? How could I set a default value?

Thanks!
Frank

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