+1 for the SQL challenge questions.

I would add some customization for employers in the message that you send out with the challenge. I would also set the reply-to / from on the email to match the name and email of the person who is sending it out.

If I were using this for job interviews, one reason I wouldn't use it is because it might scare away potential candidates because the email doesn't appear to be from me.

Looks good! No results on the third SQL challenge?

    SELECT employees.*, departments.dept_name FROM employees
    JOIN dept_manager ON dept_manager.emp_no = employees.emp_no
    JOIN departments ON departments.dept_no = dept_manager.dept_no

(excluding to_date restriction, but still...)