Would it be fair that if one cannot cancel by email then one cannot subscribe by mail since the same risks exist (of authentication and tamper-proof) exist?
Of course, you normally can't sign up for services that require payment via e-mail anyway, since no-one credible is going to ask for enough details that they can charge you through an insecure medium like e-mail.
Of course, you normally can't sign up for services that require payment via e-mail anyway, since no-one credible is going to ask for enough details that they can charge you through an insecure medium like e-mail.