Not sure how you're sending the email request, but here's a script for a button that takes its subject from a text field and takes its To: address from a checkbox with an address for the on value and empty string for off value.
var subject = TextField1.rawValue;
var email = (Checkbox1.rawValue == null) ? "" : CheckBox1.rawValue;
event.target.mailDoc({
bUI: false,
cTo: email,
cSubmitAs: "pdf",
cSubject: subject
});