Are you talking about distributed environment, where network partitions can occur? If yes, then there's Two Generals Problem and "FLP result", that just prove it impossible. So I guess you're talking about non-distributed environment.

In other words, to reliably agree on a system state (whether message id was delivered) you need the system to be Consistent. And per CAP theorem, it cannot be Available in presence of Partitions.

So other people you're referring to probably talk about distributed systems.