[user5994461]

Authentication cannot be bound to any email or format.

For instance when using google authentication, the user and the email can be anything, because google apps support custom domains for paying customers.

If you want to rely on emails, just use emails.

[icebraining]

For instance when using google authentication, the user and the email can be anything, because google apps support custom domains for paying customers.

That's what delegation is for.