LOL. Yes, of course, the ghosted verb in that phrase is "was." And it's passive construction. And normally I would argue that it doesn't matter because we're not aiming for a Pulitzer Prize in literature. But you're also right that there are reasons passive construction is usually undesirable (except in certain circumstances, as when the character is actually being a passive victim in the story). It's clearer to attribute actions to someone or something, in this case, "the system."
So I agree with both you and Tim that mentioning "the system" or just "system" (again, not aiming for great literature) in order to state exactly what happened.
The only nagging little thing is that in the Message Activity log, it does make more sense to me for the Message to be the first word. The other thing is "the system has rejected" does not feel like quite the correct tense. I'd say "The system rejected..."
Either way, "Rejected message" has got to go. ;)
Messages are the sole opinion of the author, especially the fishy ones.