'Registers an Authenticator with the ESMTPClient. The ESMTPClient will
attempt to login to the SMTP Server in the order the Authenticators are
registered. The most secure Authentication mechanism should be
registered first.
@param auth: The Authentication mechanism to register
@type auth: L{IClientAuthentication} implementor
@return: L{None}'

def registerAuthenticator(self, auth):

    self.authenticators.append(auth)
