You may want to send Intercom a user id (a unique user identifier that won't change) instead of, or in combination with an email address.
One reason to do this is that while the user's email address is subject to change in most apps, you can control the unique identifier that is sent to us, therefore maintaining a consistent user record in Intercom even if the user changes their email address.
Another reason to use the user id is if you are very concerned with user privacy and do not want to send an email address at all (just remember that if you don't send an email we won't be able to send email messages for you).
If you use phone numbers, and not email addresses, to identify your users, user_id acts as a unique identifier
Note: To update a customer's user_id in Intercom you must use the REST API.