Messaging is a protocol level abstraction, communication between entities [1]. A function call is a much lower level abstraction that does a <verb> to/with the data. One could have an entity that executes one function or many, or it could manage a whole sequence of events.

The Kay Object (KO) is more of an encapsulated microservice, like an Erlang process.

How would mail (SMTP) look if it was implemented via RPC?

[1] I specifically didn't use the word object because the Kay Object and the Java/C++ object are not comparable.