Currently, as far as I can tell, there is no way to directly query a user by email address, only by ID.
If you only have a users email address, you must first query for all users, then filter the results by the email address.
This is particularly frustrating when it comes to using the API to assign a task to a user. Currently I have to query Monday.com for all users, then filter by email address and get the users ID. Then I can use the ID to set the people column value to the user.
As far as I am aware all users are required to have a unique email address, so it seems sensible that it should be possible to query by email address.