We have just spoken to Network. They have had an issue internally with their system with the return forms, and they are now in the process of fixing it.
They have a copy of all the returns files (RTD's) that have been sent.
What they intend to do is once the problem is fixed, they will resubmit all the return files that are being rejected now, and newsagents should receive the proper email response.
Anyone who submitted a return form from late yesterday will not receive a response from Network until the problem is fixed as they have turned any processing off.
They expect the problem to be sorted some time today.
People who got the rejected message (invalid file format) should ignore it and wait for the correct response in the next day or so.