When a user is removed from a project, all connected to him or her issues (as Assign, Reporter or Watcher) stay in the project.
Users that have been removed will be shown in the project as their email address and not as a named user.
Such users can be found by filtering with one of the sections (Assign, Reporter or Watcher).
Use the “Multi edit” tool to reassign removed user’s issues to others or delete them.
Don’t forget to reassign issues with Closed status.