The Message-ID must be unique worldwide: there would otherwise be collisions between messages with the same Message-ID and the news server would reject the second article because it thinks this article has already been received.
O Message- ID ha de ser único en todo o mundo: se non, habería colisións entre as mensaxes co mesmo Message- ID e o servidor de novas rexeitaría o segundo artigo porque pensa que xa se recibiu.