USPS left a notice for TSC??

I sent in my doc by USPS express and got a tracking info as follows:

We attempted to deliver your item at 11:49 AM on August 16, 2007 in MESQUITE, TX 75185 and a notice was left. A second delivery attempt will be made. If unsuccessful, we will hold it for five business days and then it will be returned to the sender.


The address is a PO box, how come it is not delivered?