You need to pay the employees who are working on their email from home. You could however set up a policy that states that they need to have supervisory approval to do so. If they fail to have approval you still have to pay it, however you now have a disciplinary problem and can write the employee up (other other appropriate action) for breaking policy.

An non-exempt employee told me she checks her inter-office email messages from home (she has a VPN connection) on her off time, including vacations. No one tells her to do it, she said she just wants to keep on top of things while she’s out to make sure things in her area have no problems.

I’ve already told her she should not be doing that, but what do you suggest we tell her beyond this statement? “Our company does not want you to check company-related emails on your own time. If there is an emergency, someone from the company will contact you for assistance, which is compensable time for you.”