Completeness in the sense that you know if you are missing any messages that have already been sent. If all messages are logged on the blockchain, then you can

Completeness in the sense that you know if you are missing any messages that have already been sent. If all messages are logged on the blockchain, then you can see which ones you never received. Verifying completeness requires a blockchain of one type or another, where new transactions/messages reference a hash of past transactions/messages in some way.