MS sql query value one table but not another join table where*
Assuming the following table structures and that you want to return students that have NOT registered for CourseID 11...you need to change your LEFT JOIN to include CourseID = 11 and then add CourseID is NULL in the WHERE clause to pull the students that are not registered for that course. 

In your query having CourseID = 11 in your WHERE clause limits the results to the students that have registered for that course.

CREATE TABLE #AllStudentsEmail (StudentID INT, StudentName VARCHAR(20), StudentEmail VARCHAR(20))
CREATE TABLE #Course (CourseID INT, StudentID INT)

INSERT INTO #AllStudentsEmail VALUES (1000,'Joe T','jt@blank.edu')
INSERT INTO #AllStudentsEmail VALUES (2000,'Jill B','jb@blank.edu')
INSERT INTO #AllStudentsEmail VALUES (3000,'Tommy S','ts@blank.edu')
INSERT INTO #AllStudentsEmail VALUES (4000,'Sally J','sj@blank.edu')

INSERT INTO #Course VALUES (9,1000)
INSERT INTO #Course VALUES (10,1000)
INSERT INTO #Course VALUES (11,1000)
INSERT INTO #Course VALUES (9,2000)
INSERT INTO #Course VALUES (10,2000)
INSERT INTO #Course VALUES (11,2000)
INSERT INTO #Course VALUES (9,3000)
INSERT INTO #Course VALUES (10,3000)
INSERT INTO #Course VALUES (10,4000)
INSERT INTO #Course VALUES (11,4000)

SELECT  #AllStudentsEmail.[StudentID], 
 [StudentName], 
 [StudentEmail],
 [CourseID]
FROM #AllStudentsEmail
LEFT JOIN #Course ON #AllStudentsEmail.StudentID = #Course.StudentID 
    AND #Course.CourseID = 11
WHERE #Course.CourseID IS NULL

DROP TABLE #AllStudentsEmail
DROP TABLE #Course

--results 

StudentID   StudentName StudentEmail    CourseID
3000    Tommy S ts@blank.edu    NULL