Getting another users email address in Facebook app

Am developing a system for an app to award points users for recruiting their friends. When the friend accepts the request I can retrieve the id of the person who invited them but can not find their email.

Have tried:

    $fbInviter = $facebook->api("/$inviterId", '开发者_如何学CGET');
    $fbInvEmail = $fbInviter['email'];

but it appears ['email'] isn't in the returned array. The app requests permission from each user when the first access it aswell as requesting offline access so permissions shouldn't be a problem.

Any ideas?

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There is no way to get the email address for a Facebook user other than the current user, and the current user's address is only available if they've granted you the email extended permission