I believe you would have to modify (or extend) the user datatype in order to achieve that, that's kernel modification in PHP, and a bit risky. Optionally, you could modify the site.ini setting:
[UserSettings]
RequireUniqueEmail=false
and fill e-mails with whatever, for example noemail@noemail.net But mind that you're loosing some system functionalities this way.
Thank you for your answer.
I have already put <i>false</i> to this setting.
Actually I just need to bypass the verification of the email input, if it is empty or not.
It's in the user datatype ?
Or which kernel file do this ? Thank you very much.