RE: Backup MX records.

  • From: "Mulnick, Al" <Al.Mulnick@xxxxxxxxxx>
  • To: "[ExchangeList]" <exchangelist@xxxxxxxxxxxxx>
  • Date: Wed, 26 May 2004 11:08:29 -0400

Generic SMTP problem.  4xy is a transient error (specifically, 421 is
transient indicating that the host might be shutting down and must end the
transmission).  It's not a hard error, so it's still a valid mail handler.
If you have done the resolution, then you must then use the host specified.
In this case, it's the lowest cost and you will therefore use that if it's
available.  If it's not available for some reason, (off network and not
answering at all) then you can use a higher cost MX record.  
General rule: if it's on the network answering SMTP verbs, it needs to be
able to handle the traffic.  If it can't, it needs to be removed to correct
mail flow. From 974:
"For example, a response code of
   "non-existent domain" should probably cause the message to be
   returned to the sender as invalid, while a response code of "server
   failure" should probably cause the message to be retried later"
It's not a hard and fast rule, but if all hosts were to repeatedly try the
mailers for a transient error, there would be significantly more traffic on
the internet today.


From: paul_lemonidis@xxxxxxxxxxx [mailto:paul_lemonidis@xxxxxxxxxxx] 
Sent: Wednesday, May 26, 2004 10:45 AM
To: [ExchangeList]
Subject: [exchangelist] Backup MX records.

Hi All
I have an Exchange 2003 Service Pack 1 machine.
My question relates to SMTP sending. Every so often I see messages queued up
for remote hosts. Sure enough from the machine I telnet to the lowest cost
MX record for the remote host and the connection is dropped with an error
421, connection lost message. However, on a number of occasions I can
sucessfully telnet to the higher cost hosts. Exchange, however, will simply
not use them. Today as a test I decided to setup a second Internet Mail
connector and limit the address space to just the Domain affected. I then
rather than use DNS to forward the message chose to use the second MX record
to forward to the second lowest cost MX record host directly. I then went to
the queue and forced delievery. This time the message was delievered fine,
well the message certainly disappeared from the queue and was not bounced. I
will not be absolutely able to confirm he received it until tomorrow
unfortunately but I am confident he will have.
My question is this. I was told many months ago that 421 means please try
again later and hence Exchange will do that rather than trying alternative
MX records. Does this thus mean that any server on the Internet that issues
this message makes the whole concept of backup MX records worthless? Based
on what I understamd you could have 100 servers but if the first one has the
lowest MX record cost and issues a 421 message Exchange\SMTP will simply
ignore all 99 others despite them being able to receive perfectly? Firstly
am I correct and secondly is this a generic SMTP problem or an Exchange
problem and finally other than my somewhat less than ideal workaround is
there a better way of resolving the issue please?
Many thanks in advance.
Paul Lemonidis.  
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