The Post Office and Postmasters have always had discretionary right to accept or refuse mail -- to CENSOR what is acceptable, to mail or deliver, by their personal morales. Here's a perfect example.

At the turn of the 20th century, a young photographer who owned a post card shop in Lebanon, Pa., by the name of Luther G. Harpel, had been taking pictures at a local Chautauqua/Resort. One day he saw a group of young girls sitting at the beach. He asked a young teenage girl if she would pose for some tasteful post card pictures. She and her friends laughed. She ran and got her mother's approval. Everyone, including the girl, knew Harpel and his picture postcards; four tasteful penny post cards were processed for sale -- none revealing anything or showing the girl's face
But somebody didn't approve. Her father didn't approve -- he thought the four images to be immoral -- and he also happen to be the local Post Master! It was reported by his assistants that he personally confiscated and destroyed every card mailed at the Mt. Gretna Post Office.
Early publications, like the "Postmaster's Gazette", carried articles about the type mail local Post Masters were not permitting to be mailed or delivered. It was done then. It will continue to be done now and in the future.
Free speech is never free and some never will be; inciting a "Lynch Mob" mentality is one of them and branding people with "Scarlet Letters" went out of fashion centuries ago.