insufficient postage.
Incorrect address, or perhaps the recipient no longer lives there, or is dead.
If you affix postage and postmark from a private postage meter, and don't send the letter after two or more days, you may have the mail sent back for "delaying the mail." This one I am worried about as I just dumped quite a bit of mail two days after postmarking from my office postage meter. I would really look foolish if it all comes back to the office. But the postman assured me he would take care of it. I hope so.