john.doe@gmail.com
johndoe@gmail.com
A better approach might be to limit the effect that a new account can have in the system. Hackernews, Reddit, Stackoverflow all do this through reputation. A new account on these systems is unable to achieve much until time is spent proving the account is being used. Thus reducing the incentive for an individual to create multiple accounts.
Which is impossible; a number of people run their own email or use a small friend-run email server, and you can't possible discern the delivery rules from the outside.
Yes. A small number is a number.
If we could go back in time and force every single implementer to follow every relevant RFC to utter perfection (and make sure all the RFCs were perfectly unambiguous), I'd be more sympathetic.
But email is fucked. The sheer number of oddball things, hacks, workarounds, deviations and other bits of mess that implementers have engaged in over the years means the RFCs should be treated as at best a loose hopeful outline of how email might in theory work.