Let's say you want to advance this project later, to have permissions and access levels. So you have a usergroup, that user with emailid 'example@no-host.com' is a member of, lets him login on your site with his email and manage some things on list as an administrator.

Then he changes his email, woopsie, you have to manually change this cause there's noone with the emailid of 'example@no-host.com'.
Thus, an emailid as a numeric, and keeping the actual email in another field, is the better solution, as I said; It will open more doors for future use.

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