Rick Rothstein and I had similar experiences recently: posting responses
that failed to appear on the MSNews server. Of course, there can be many
factors that be the cause.

But in our cases, the problem exists on the MSNews server. I will explain
in more detail below. But the upshot of it all is....

Do not include the characters x y z (any capitalization) without
interstitial spaces, either in the subject or message body.

The MSNews server deletes any message that contains that character sequence.

The deletion usually happens immediately after receiving the message, either
from a newsreader or from another news server. But sometimes, the deletion
occurs minutes later. Consquently, the message might or might not get
propagated to other servers before it is deleted on the MSNews server.

For the same reason, the message might appear momentarily on the MSNews
server, allowing you to read it. And if you use Outlook Express, Windows
Mail or any other newsreader that maintains its own message archive on your
computer, you might think the message persists longer "on the MSNews server"
than it really does simply because it is on your computer.

But someone else might not see your message on the MSNews server because
they looked later. And if you reset and resynch the NG, the message will no
longer appear. Another symptom is that you might get a "removed from
server" error when you try to read a message whose header is already in the
newsreader archive on your computer.

Originally, I misinterpreted the deferred deletion as an "intermittent"
failure to post the message. But the behavior is actually consistent.

Also, I had conjectured that this "filtering" (deletion) was related to
messages or responses to messages that originated on Google Groups. In the
final analysis, that has nothing to do with it.

Note: I can only confirm this behavior when the x y z character sequence
(without interstitial spaces) is in the subject or message body. But I
suspect it can also be triggered when the character sequence occurs in any
message header, which includes random character sequences that we have no
control over.

I come to these conclusions based on a careful analysis of the NNTP protocol
in network traces.

The network traces reveal that the MSNews server always receives and accepts
the messages. They also reveal that the news server assigns an article
index to the message before it is deleted. And the network traces
demonstrate that the messages are deleted sometimes in less than 0.2 sec and
sometimes after more than 5 min. Finally, in a sampling of 28 deleted
messages, only about 11 of them were propagated to the Google Groups server.

(I presume that they were also propagated to other news servers, e.g. the MS
Discussion Groups server. But I stopped monitoring the MSDG server because
the propagation delay is typically 30-35 min, at least in recent days.)

The deferred deletions and inconsistent propagation to other news servers
suggests to me that the messages are always stored on the MSNews server
initially, and independent separate processes forward and delete the
messages afterward.