Currently it is not possible to retrieve email of user within the Org in case email visibility is set to "Only you and admins".

There are a couple of API endpoints, however, they are not working (expectedly) 

1. [{{{{{}baseUrl{}}}}}/rest/api/3/user?accountId=...] This doesn't work due to visibility of email
2. [{{{{{}baseUrl{}}}}}/rest/api/3/user/email?accountId=...]. This doesn't work because it's a user making the call not a whitelisted application.

There should be API to retrieve user email in case it is called with admin authentication (admin has permissions to view email).

Use case scenario: Implementation of user picker fields in JSM forms which should query list of users.