Add substitutions to the email

        :param value: Add substitutions to the email
        :type value: Substitution, list(Substitution)

def substitution(self, substitution):
        """Add substitutions to the email

        :param value: Add substitutions to the email
        :type value: Substitution, list(Substitution)
        """
        if isinstance(substitution, list):
            for s in substitution:
                self.add_substitution(s)
        else:
            self.add_substitution(substitution)