Backup MX records.

  • From: <paul_lemonidis@xxxxxxxxxxx>
  • To: "[ExchangeList]" <exchangelist@xxxxxxxxxxxxx>
  • Date: Wed, 26 May 2004 15:44:34 +0100

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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