This song is about a guy who sends a letter to a girl, but she refuses to read it, instead writing "Return To Sender" on it and having it sent back to him. Our hero has a hard time believing she doesn't want to read the letter, so he sends it special delivery to make sure it arrives. When that letter gets sent back, he decides to hand-deliver it.
The year after this was released, postal "Zones" were replaced by Zip Codes, making the line, "No such number, no such zone" outdated.
Otis Blackwell, who had written Elvis' hits "Don't Be Cruel" and "All Shook Up," wrote this with Winfield Scott, who was in a vocal group called The Cues and wrote the song "Tweedle De" for LaVern Baker. When Elvis needed songs for his movie Girls! Girls! Girls!, Blackwell convinced Scott to try his hand writing Pop songs, and they came up with this and "We're Comin' In Loaded" for the movie.
On January 8 1993, the US postal service released a stamp commemorating Elvis on what would have been The King's 58th birthday. This caused a stir, as stamps were usually reserved for historical figures rather than entertainers, so the postal service let their customers vote on one of 2 designs for the stamp: a young Elvis or an older (and heavier) Elvis. Young Elvis won handily and on January 8, 1993, the stamp was released. Enterprising stamp collectors put Elvis stamps on letters that day and mailed them off with false addresses so they would be sent back marked "Return To Sender" and become collector's items.