I am the original poster of this question, we have been telling employees to add their box number after the address.  For example, 123 Main St #56, Anytown, CA 99999.  I hope this helps!  We also have no mail delivery in our area and I think it is impossible for people to grasp that this is an issue in rural areas.  I can consult the IRS all I want, but it is not going to get our post office to deliver mail to employee's homes.