Question
Is there a user id that is not the user's email address?
Answer
In Zendesk users are provided with two types of id's, not related to the email address: the user ID and the user external_id.
id
The id of a user is part of the URL of a user's profile:
https://subdomain.zendesk.com/agent/users/{user_id}/assigned_tickets
User id's are also available via the User API:
https://subdomain.zendesk.com/api/v2/users/{user_id}.json
"id": 1987389842
external_id
The external_id is a unique identifier coming from another system, that allows mapping between Support and third-party integrations. Like the id, this attribute is also available via the API, and null by default.