But that's only if someone knows that your email is bob@duck.com, right? (And the same would happen if someone knew your true email address was bob@example.com

But that's only if someone knows that your email is bob@duck.com, right? (And the same would happen if someone knew your true email address was bob@example.com - they could just send emails there.) I think the intended use case is that you use a different and unique alias, e.g. randchars83@duck.com, for every service. At least, that's how we intend Firefox Relay to be used. Then you can just throw away that alias if it starts getting used for unwanted messages.