[hn_urbit_thr123]

Yes, but the receiver can be faulty. If it acknowledges the message and then crashes before handling it, you've got at-most-once, and if it handles the message and then crashes before acknowledging it, you've got at-least-once. You can avoid this if the receiver handles and acknowledges in a single transaction, but I only know of one platform that implements this and everyone hates it (hence the throwaway).

[mountainreason]

Even with a transaction, if the processing involves external side effects, e.g. sending and email, the rollback won't matter and you still get at least once.

[hn_urbit_thr123]

There's no rollback, it's an atomic transaction. The certainty that messages are always handled completely or fail completely is one of the big design constraints that made the whole thing so hinky.