Frank Bures wrote on 04/02/2009 02:24:50 PM:

> Further to my query - detailed investigation revealed a disturbing
behaviour:
>
> When LISTSERV passes the message to sendmail, it creates blocks of
> addressees containing up to 100 entries.  In my particular case, I had 8
> such blocks for some 750 subscribers.

This is normal behavior - breaking it up into blocks of 100.  This is
controlled in the site configuration.

> When the duplicate mailing occurs, LISTSERV passes one (and just one) of
> those blocks to sendmail again, some 10 minutes after the original
mailing.
> The block always contains external addresses.  They do not have to be
> related to our central university mailing system.  Duplicate mailings
> occurs intermittently to yahoo and other universities as well.  In other
> words, any addresses that are in that single block of 100 entries will
be
> re-mailed.

It sounds like the sendmail server is not confirming the receipt of the
message with a 250 Message Accepted, so Listserv is not sure it went
through and retries the message.  Is the sendmail system under your
control or someone else's?  Is that system heavily loaded?

Worst case, you might need a sniffer (or tcpdump) on the network
connection