G'day all

I'm seeing the most bizarre behaviour that I've ever seen with LISTSERV.
When a post hits one of our lists it appears to get resent to *most* of the
recipients without any problems except for a few.  For these few, it appears
that the mail server gets a very confused DNS response and thinks the mail
exchanger for their domain is actually our LISTSERV box.  It then happens to
the next recipient, and the next, and the next, ad nauseum.  The resulting
header starts to look like this:

Your message
        To:  [log in to unmask]
did not reach recipient
This mail domain has following registered mail servers:
 Mail eXchange host:listserv.towersoft.com.au priority:0
After command RCPT TO: <[log in to unmask]>
 Server responded:>550 Recipient not local.

and quickly grows to look like this:

Your message
        To:  [log in to unmask]
did not reach recipient
This mail domain has following registered mail servers:
 Mail eXchange host:listserv.towersoft.com.au priority:0
After command RCPT TO: <[log in to unmask]>
 Server responded:>550 Recipient not local.
After command RCPT TO: <[log in to unmask]>
 Server responded:>550 Recipient not local.
After command RCPT TO: <[log in to unmask]>
 Server responded:>550 Recipient not local.
After command RCPT TO: <[log in to unmask]>
 Server responded:>550 Recipient not local.
After command RCPT TO: <[log in to unmask]>
 Server responded:>550 Recipient not local.
After command RCPT TO: <[log in to unmask]>
 Server responded:>550 Recipient not local.
After command RCPT TO: <[log in to unmask]>
 Server responded:>550 Recipient not local.
After command RCPT TO: <[log in to unmask]>
 Server responded:>550 Recipient not local.
After command RCPT TO: <[log in to unmask]>
 Server responded:>550 Recipient not local.
After command RCPT TO: <[log in to unmask]>
 Server responded:>550 Recipient not local.
After command RCPT TO: <[log in to unmask]>
 Server responded:>550 Recipient not local.
After command RCPT TO: <[log in to unmask]>
 Server responded:>550 Recipient not local.
After command RCPT TO: <[log in to unmask]>
 Server responded:>550 Recipient not local.
After command RCPT TO: <[log in to unmask]>
 Server responded:>550 Recipient not local.
After command RCPT TO: <[log in to unmask]>
 Server responded:>550 Recipient not local.
After command RCPT TO: <[log in to unmask]>
 Server responded:>550 Recipient not local.
After command RCPT TO: <[log in to unmask]>
 Server responded:>550 Recipient not local.
After command RCPT TO: <[log in to unmask]>
 Server responded:>550 Recipient not local.
After command RCPT TO: <[log in to unmask]>
 Server responded:>550 Recipient not local.
After command RCPT TO: <[log in to unmask]>
 Server responded:>550 Recipient not local.
After command RCPT TO: <[log in to unmask]>
 Server responded:>550 Recipient not local.
After command RCPT TO: <[log in to unmask]>
 Server responded:>550 Recipient not local.
After command RCPT TO: <[log in to unmask]>
 Server responded:>550 Recipient not local.
After command RCPT TO: <[log in to unmask]>
 Server responded:>550 Recipient not local.

A quick scan of the logs confirms that the individual mails are attempting
to be sent right back to LISTSERV where it naturally generates an error to
me.

There seems to be some correlation between the alphabetical order of the
recipient domains.  (Is this how LISTSERV decides where to split bundles of
subscribers into smaller files?)  Apart from that there's no discernable
similarities between occurances.  If it applied to the same list or
subsriber(s) I'd have someplace to look for an answer but I've no idea where
to go next.

I've checked our internal DNS - the MX records are correct.  I've checked
our external DNS - same thing.  I've checked with our ISP's DNS -
everything's in order.  I did an NSLOOKUP to try and find a prority=0 mail
exchanger (as seen in the header) with no result.

Is anyone able to help?  Is it LISTSERV that's the culprit or am I barking
up the wrong tree?  Any suggestions are welcome.  Feel free to reply
off-list if you wish.

Chazzozz!!

Michael Shannon
Webmaster
[log in to unmask]

"Nature has provided us with two ears, two eyes, and one tongue, to the end
that we should hear and see more than we should talk." - Socrates

Note: Opinions expressed on this list are my own and do not reflect the
views, opinions or position of my employer.  If swallowed, seek medical
advice.