Can I use Priority to mail application to USCIS

Why the delivery status shows

We attempted to deliver your item at 1:18 PM on December 7, 2006 in MESQUITE , TX 75185 and a notice was left. It can be redelivered or picked up at the Post Office. If the item is unclaimed, it will be returned to the sender. Information, if available, is updated every evening. Please check again
later.

I didn't use signature confirmation service. How come to get this status???