On Fri, 26 Feb 2016 09:38:43 -0500, Paul wrote:

The Message-ID or MID, must be guaranteed to not collide
with the MID of any other message. If your NNTP client
software accepts the "suggested" MID value coming from the
server, then it's up to earthlink in this case, to use
a pseudo-random number generator, to generate a MID.

My understanding, and I could well be wrong, is that NNTP clients are
allowed to generate a MID. If they do, then the server *may* honor it, or
the server may replace it with a server-generated MID. If the client does
not generate a MID, then the server *must* do so.

I don't think there's a scenario where the server "suggests" a MID back to
the client. The mechanism is simpler than that. The server has no idea that
you're creating a new message until you actually submit it. By that point,
either the client generated a MID or it didn't, and the server simply reacts
to that, on the server itself and not by engaging the client in any way,
before completing its processing of the message.

But the news client software, is also allowed to generate its
own MID.

By default, messages created with Forte Agent use the 4ax.com domain, for
example, but in the Agent newsgroup I've seen methods posted where the user
can change it to anything they want. While they're at it, they can change
most of the other headers, as well, creating their own custom formats and
text strings.