There are several reasons why a letter can be returned as Undeliverable:

1 - No house number or uncertain. This sometimes is the case on Indian Reservations. Some tribes don't name their streets or use house numbers.

2 - Locked outer gate.

3 - No name on mailbox. Some years back, the postman informed us that if our box did not list the last name of everybody at our house who was to receive mail, he would no longer deliver it. We promptly taped a piece of paper with the names on the box, mail delivery resumed.

4 - "Beware of Dog" sign in fenced yard. Carriers don't dare enter. Especially if they've had a bad encounter with the dog before. Even if it's a schnauzer.

5 - Route to mailbox is too dangerous - overhanging tree limbs, that fake but convincing hand grenade you left beside the front walkway. Ha-ha-ha!
Or a past threat made to the carrier: "Deliver one more piece of junk mail here and I'll kill you!" Or the only road to your house goes over that quaint wooden bridge . . . which just washed away in a flood.

6 - The piece of mail itself is too dangerous to deliver. My uncle once ordered a colony of bees. The little critters go out and buzzed around the post office. The postmaster called him and said, if you want your stupid bees, YOU come down here and pick them up. ALL OF THEM.

7 - A postal strike or war has broken out between the sender's country and the recipient's country. Pam, did you declare war on your kids?

8. And then . . . there is the unexplainable. Not long ago, I mailed two envelopes containing a couple of chips in each, to a collector. They were mailed at the same time from the same collection box, with the same postage prepaid. The first one got through with no problem. The second one never made it; returned to me Undeliverable/Unable to Forward/Postage Due. Carl Shalit can only get one letter a day?