RE: Backup MX records.

  • From: <paul_lemonidis@xxxxxxxxxxx>
  • To: "[ExchangeList]" <exchangelist@xxxxxxxxxxxxx>
  • Date: Wed, 26 May 2004 16:34:47 +0100

Hi Al

I see your point about traffic levels. The problem is I have seen mail queue up 
to these Domains for several hours at a time or even timeout after the deafult 
2 days? We also have a couple of clients to whom we constanty have this 
problem. Could this possibly be a firewall issue at my end? I guess it could by 
the same token be their firewall but if they are receiving mail from everyone 
else I think that is unlikely. I have a Watchguard box and what they do with 
SMTP is unbelievable. Even NDR's back to senders are heavily modifyied! What 
Exchange sends and what gets out the firewall are totally different!!  

From what you say about resolution the force connection option must cause DNS 
to be queried again as once I had my second connector up and clicked the option 
the  message in the queue form before it was created was delievered.

Many thanks for your prompt reply and help.

Regards,

Paul Lemonidis.
  ----- Original Message ----- 
  From: Mulnick, Al 
  To: [ExchangeList] 
  Sent: Wednesday, May 26, 2004 4:08 PM
  Subject: [exchangelist] RE: Backup MX records.


  http://www.MSExchange.org/

  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.

  Al



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


  http://www.MSExchange.org/

  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.

  Regards,

  Paul Lemonidis.  

    
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