How to fetch email ids of users that do not have the same domain name as that of the confluence site

[Deleted user]

I am using the API  : 'rest/api/content/{page_id}/restriction?expand=restrictions.user,restrictions.group,content' to get the email ids of all the users associated with a page. However i have observed that for users outside the email domain name used while creating the confluence site is getting retrieved as an empty string. 

For Instance:

I created the site using himanshi@abc.com email. So i am able to retrieve emails of all users with @abc.com email but not able to retrieve users with other domain emails like @Gmail.com. An empty string is getting retrieved for such emails.

Am I doing something wrong here or is there a way I can get these other email ids as well?