Facebook Connect API Email issue

Question

I have some issue with the Facebook Connect API

I can't understand why for some users, I can't get the email field.

It working for some, and other not.

I did some debug to check the permissions :

$response = $facebook->api( "/me/permissions");
print_r($response, true);

and then I receive :

[data] => Array
        (
            [0] => Array
                (
                    [installed] => 1
                    [basic_info] => 1
                    [public_profile] => 1
                    [email] => 1
                    [user_friends] => 1
                )

        )

So the email permission is set but when I run:

$user_profile = $facebook->api('/me');

there is no email index in the array...

Thank you in advance for your help!