Hi Justin,
Another way to come at this is to occasionally pull that user formula value over into a static field. For example with an Automation that when a new record is created on that table and the User formula field has a value write the value from that formula field to a static User field. That way the user at that time is captured in the field and when the user changes email addresses later it will update as a standard user field would instead of the issue you are running into with the formula field. Then that formula would just exist in the background to populate that field when the record is created. I hope that suggestion is helpful.
------------------------------
Evan Martinez
Community Marketing Manager
Quick Base
------------------------------