Anybody else getting multiple (sometimes a dozen) copies of same mail from this list?
Looks to be only related to yahoo users. It seems clear to me that the yahoo mail servers are at fault. From the mail logs:
Jan 3 08:43:59 intrepid postfix/smtp[4640]: 919DA255DB: to=kd4tjx@yahoo.com, relay=mta5.am0.yahoodns.net[66.196.118.35]:25, delay=47, delays=1/15/16/15, dsn=4.4.2, status=deferred (lost connection with mta5.am0.yahoodns.net[66.196.118.35] while sending end of data -- message may be sent more than once)
This means that while delivering the message to their servers, the connection was terminated by the remote end. This is a clear violation of SMTP rules, where the receiving end needs to give an ACK (200 code) or NAK (400/500 code) to indicate whether it has taken possession of the email or not. Without such a response, my server has no idea if the remote side has it or not, so it resends it to make sure that it is not dropped (another requirement of the SMTP rules).
It's clear that it's not even a mis-configured SPAM filter, because the message *is* being delivered to the user even after the terminated connection (which is why they end up with duplicates).
There are 1446 such failures in the logs since 2013-12-29, all from yahoo mail servers. If you're a yahoo user, I suggest you take it up with them.
--Dan