def unenroll_by_email(cls, email, course_id):
        """
        Unenroll a user from a course given their email. This saves immediately.
        User lookup errors are logged but will not throw an exception.

        `email` Email address of the User to unenroll from the course.

        `course_id` is our usual course_id string (e.g. "edX/Test101/2013_Fall)
        """
        try:
            user = User.objects.get(email=email)
            return cls.unenroll(user, course_id)
        except User.DoesNotExist:
            log.error(
                u"Tried to unenroll email %s from course %s, but user not found",
                email,
                course_id
            )