Would you consider it rude to not at all acknowledge a gift sent by mail? It think it's rude AF because I don't even know if the receiver actually got it.

I've only had young cousins (both mine and DH's) do this. One was a graduation gift, an initial necklace and the other was 2 sets of baby clothes. My cousin eventually responded months later (probably because my aunt bugged her) with a lame excuse about wanting to send a card but she didn't have my address. DH's cousin never even bothered responding when after I asked if she gotten it. I was worried they didn't fit and even offered to get a larger size, but no response at all. I'm not sure if it got lost in the mail or what.

Has this happened to you?