I don’t know if I understand what you mean in your question, but I will mention that to the Facebook documentation for permissions:
Provides access to the person’s primary email address via the email property on the user object.
Do not spam users. Your use of email must comply with both Facebook policies and with the CAN-SPAM Act.
Note, even if you request the email permission it is not guaranteed you will get an email address. For example, if someone signed up for Facebook with a phone number instead of an email address, the email field may be empty.
for reference: https://developers.facebook.com/docs/facebook-login/permissions/v2.0
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solved Error while logging in with facebook if the account contain second email [closed]