Mailing address is out of whack

I had sent a letter with money order to pre-reg space in your art show. AND IT CAME BACK!! The letter came back to me last thursday with a note saying that the p.o.box was closed, unable to forward letter, return to sender. I did send it to address listed on web-site Box 310, Deer Park, Texas,. Just courious as to what is going on. Your friend allways, Donald P. Welch. Thank you.