def add_to(self, email):
        """Add a single recipient to this Personalization.

        :type email: Email
        """
        if email.substitutions:
            if isinstance(email.substitutions, list):
                for substitution in email.substitutions:
                    self.add_substitution(substitution)
            else:
                self.add_substitution(email.substitutions)
        if email.subject:
            if isinstance(email.subject, str):
                self.subject = email.subject
            else:
                self.subject = email.subject.get()
        self._tos.append(email.get())