If the USPS cannot deliver your mail as addressed and you've not added an ancillary endorsement, it will follow these guidelines, based on mail class:

Priority Mail and First-Class Mail:     Same as USPS action for "Forwarding Service Requested". The mail will be returned to the return address on the mailpiece. The length of time it takes for a piece to be returned varies greatly.
Standard Mail:                                  Mail piece disposed of by USPS