Mail is Something Else Not Running During Polar Vortex
Tons of things aren't running today and tomorrow (Wednesday-Thursday) due to the sub-zero temps and you can add mail to the list.
According to WILX, The U.S. Postal Service (USPS) has suspended mail service Wednesday and Thursday throughout the state. USPS reason for not delievering mail are mostly due to safety as record low wind chills are being reported. And with mail carriers having to be outside a lot, the USPS isn't willing to risk the safety of their employees. Even though the mail isn't coming to your place over the next two days you can still go to their retail sites as they will be open. But be careful driving there if you are going there.
So add this to the list of the many places closed due to the cold. You can actually check and see what departments are closed across Mid-Michigan here.