LiveCycle Designer - How do you stop the "Submit by E-mail" button to generate the email if the Signature Field object is not signed but allow to generate when signed?

I am trying to stop the Submit by Email button in LiveCycle Designer from generating the email to send when clicked, if the Signature Field on the form is sign then it will be able to process normal.

How do I complete the formula?

So for this is what I have:

var oState = event.target.getField("Form1[0].Main3[0].SignatureField[0]").signatureValidate();  

if (oState != 0)  

{ 

     EmailSubmitButton.execEvent("click");

} 

else 

{ 

     xfa.host.messageBox("You have not signed the form. \n\nPlease click OK and then click the signature field to sign the form. \n\nThank you!", "Error submitting this form", 0, 0);   

  Form1.Main3.EmailSubmitButton.validate = "disabled"

}

Thanks