The 430 messages originate at the Newshosting end.
I can no reason why it should be dependant on the client software used. Newshosting should either have the post requested or it does not (and thus the 430 error).
Are you sure that Newsbin is actually connecting to the same server at the Newshosting end as when you try Grabbot? It is possible this depends on what Newshosting server you actually connect to if there servers are not all in syn.