I've reviewed the service issue on the PlusNet web-site & found this
'snippet':

"I don't think it's entirely fair to put all of the blame at our
doorstep. Let's put things into perspective. This spam protection has
been in place since we implemented Postini. Nothing has been changed at
Postini's side. Earlier on today we relaxed a certain area of the spam
filtering (at a risk to our mail delivery platform) and the problem now
appears to be resolved.

Why did uk2.net's servers all of a sudden start triggering this refusal
to accept messages despite none of the thresholds being changed? - That
I'm afraid I cannot answer as we struggled to get a response after
raising support tickets (and the two telephone coversations we had with
them didn't achieve too much either).

I don't think it's explicity any parties fault, and some of the blame
definitely has to lie with us. With better cooperation from the parties
involved though (Postini included), I reckon we could have nailed this a
little sooner."