let's not lose sight of the principle, which is consent, while we deal with methods, like authentication.
Full agreement. Five basic Human e-Rights seem to be to e-exist, to e-own, to e-send and e-receive what one wants, to e-associate. The principle should be "I have a mail for you, may I forward it?", probably best implemented in the day to day life as "I have mail for you, you may want to retreive".