Is there a user id that is not the user email address?
In Zendesk Support, users are provided with two types of id's, not related to the email address: the user id and the user external_id.
The id of a user is part of the URL of a user's profile:
User id's are also available via the User API:
It's a unique identifier coming from another system, that allows a mapping between Support and third-party integrations. Like the id, this attribute is also available via the API, and null by default.