Email includes the email address of the stated authors if you want to indicate them. It's not a common field and might not be supported.

To specify email addresses, the following variants are recommended:

As a note to display at the end of the reference:

@book{ ... ,
 author = {Muller, John},

 ...

 note = "{\tt john.muller@example.com}"
}

To display behind the family name:

@book{ ... ,
 author = {Muller {\tt john.muller@example.com}, John },

 ...

}