Currently, this is not feasible.
We have the following mechanisms in the roadmap
1. Allow the user to use their actual email address (firstname.lastname@example.org). In this case, the user will have to do an OTP or third party (google OAuth) authentication. This is currently in our SDK.
2. Get user token using the user email address and an OTP. This can be used for internal users (email@example.com). Please note that this will require your application to integrate with the SMTP server to be integrated to handle the OTP emails and generate the user token based on it. The above API is under development but will be released shortly. The app SDK can then be initialized with this token and associated user id.
3. For registered and verified domains, we will have an API to generate tokens using the domain level key and secret. This will work only with email addresses belonging to same domain. The app SDK can then be initialized with this token and associated user id. This mechanims is still under development.
(2 and 3 would be available by Oct 2015 end)