I've had similar experiences Allan, when I mailed out a few of my Atlantic City Catalogs on a Sunday when the Post Office was not open and I dropped the flat-rate pre-stamped priority mail envelopes into the outdoor mail receptacle at the post office.

I spoke to my mailman about the returned mailing and he told me next time to put them in my personal mailbox with the red flag up at the curb at the front of my house for pickup. I've never had another one returned.