I think you talk about bitmessage. It doesn't use blockchain, though, which is kinda useless, there is no need to store messages permanently without the way to

I think you talk about bitmessage. It doesn't use blockchain, though, which is kinda useless, there is no need to store messages permanently without the way to remove them. These systems mostly use blockchain as PKI or similar to bitcoin mempools with public crypto, but not store messages on blockchain.