Gi'day.
        Karen Reznek mentioned "bounced
        mail" in her note of Sat, 25 Aug 2001.
        Lemme share a recent quirk of ours.

        I use a commercial ISP, but our List
        is out of ASU.  The two other ListOwners
        are located at ASU and the U of A.

        For about five weeks, I have been unable
        to communicate via my ISP to the U of A
        Owner, but I can communicate with ASU.
        Not only that, we: the ASU ListOwner
        and I received error messages about
        the U of A membership on our List.

          ----- The following addresses had transient non-fatal errors -----
        <[log in to unmask]>

        ----- Transcript of session follows -----
        <[log in to unmask]>... Deferred: Connection timed out with
        cosmo.as.arizona.edu.
        Warning: message still undelivered after 4 hours
        Will keep trying until message is 5 days old

        Not only that, eventually our ListOwner at the U of A was
        "un subscribed", by Listserve, for all the accrued messages
        not being delivered.

         I was forced to request assistance from my
        ISP Customer Service.  Eventually the problem was resolved,
        here's the explanation.

"To Mail Administrator:

I am the Senior System Administrator for (My ISP) and I
wanted to bring to your attention a problem with your <@Y.com> mail
service.

I have recently upgraded our Linux mail server to the new 2.4.x stable
Linux kernel.

The new Linux Kernel implements Explicit Congestion Notification - a new
method defined in (http://www.ietf.org/rfc/rfc2481.txt?number=2481) RFC
2481 for improving TCP performance in the presence of
network congestion by allowing routers to provide an early warning of
traffic flow problems.

Unfortunately, there are bugs in some Firewall products which cause them
to reject incoming packets with ECN enabled. I believe your company is
employing such a Firewall or packet filter on the public side of your
primary MX record (mta.Y.com), thus you are rejecting legitimate
mail service from many, many sources.

Currently in our small ISP environment (<10,000 email users) we have
queued over 40 messages destined for your mail server (mta.Y.com).
Below is a example of the mail Q listing -

##########################

f7OFBGL15955 2 Fri Aug 24 08:11 <[log in to unmask]>
(Deferred: Connection refused by mta.Y.com.)
<[log in to unmask]>

###########################

As you can see, the SMTP connect is refused from the Y mail server.
This mail server is running the Linux 2.4.x Kernel with Explicit
Congestive Notification enabled.

Fixes are available from some router vendors, and have been since at least
mid-2000. These are not "feature patches" (which may add new features and
have new bugs), but purely bug fixes, and thus should be safe to use, even
for the most paranoid.

I will include some links with more relevant information for you to
review. I would be able to assist you in any fashion you would like for
testing purposes.

RELATED LINKS:

Linux Kenel FAQ (Anchored to appropriate section)
http://www.tux.org/lkml/#s14-2

RFC 2481 (The official ECN Spec) <Please Obey>
http://www.ietf.org/rfc/rfc2481.txt?number=2481

Please let me know if it will be necessary for our ISP to disable ECN from
our 2.4.x kernel because of non RFC compliance on the part of the Excite
Network.

Thank you very much for your attention on this matter, I do appreciate
your time."

        Yes, the U of A HAD installed a new Firewall - unidentified
        manufacturer and version so far.

        I am now able to communicate via e-mail with my
        third ListOwner.  We had been telephoning about the
        problem in the mean-time.  Once the Firewall was
        corrected he received a flood of mail which had been
        building up outside the gate.

        Just something else to keep in mind when you
        receive "bounced" messages from Listmembers.

        Thank you for your time.
        Dave J.