OT: New Yahoo behaviour noticed with mailing lists

[Jim Holland]

Today I notice that mail from one of our mailing lists is undeliverable to 
Yahoo.  This is a sample sendmail SMTP transaction:

>>> MAIL From:<list at mango.zw> SIZE=3376
250 sender <list at mango.zw> ok
>>> RCPT To:<user1 at yahoo.com>
>>> RCPT To:<user2 at yahoo.com>
>>> RCPT To:<user3 at yahoo.com>
>>> RCPT To:<user4 at yahoo.com>
>>> RCPT To:<user5 at yahoo.com>
>>> RCPT To:<user6 at yahoo.com>
>>> DATA
250 recipient <user1 at yahoo.com> ok
250 recipient <user2 at yahoo.com> ok
250 recipient <user3 at yahoo.com> ok
250 recipient <user4 at yahoo.com> ok
250 recipient <user5 at yahoo.com> ok
421 Too many recipients
>>> QUIT
354 go ahead
<user6 at yahoo.com>... Deferred: 421 Too many recipients
<user1 at yahoo.com>,<user2 at yahoo.com>,<user3 at yahoo.com>,<user4 at yahoo.com>,<user5 at yahoo.com>... Deferred: 421 Too many recipients

It is rather curious.  Clearly Yahoo is adopting a new policy of not
accepting mail to more than 5 recipients at once - fair enough.  Then our
system should be able to deliver to the first five and queue the message
for delivery to the remainder later.  However our sendmail 8.13.8 is
issuing a QUIT as soon as it sees the "421 Too many recipients" instead of
sending the data to the recipients whose addresses have been accepted, in
response to the "354 go ahead".  As a result of this we have ten
undeliverable messages from the list to Yahoo, all with more than five
Yahoo recipient addresses in them.

I wonder if this is the fault of our sendmail or something unusual being
done by Yahoo (eg refusing to accept any data if a message has more than 5
recipients - but the 354 response above would seem to belie that).  It is
odd that it has not happened in the past.  I would have thought that our
system should not be issuing the DATA command until it had received the
responses to all the RCPT TO commands - it looks like we are trying to
slam commands into the recipient system without waiting for a response -
but that is the way that sendmail always behaves.  Maybe it's time to
change to Exim :-)

Is anyone else experiencing this problem?

Regards

Jim Holland
System Administrator
MANGO - Zimbabwe's non-profit e-mail service