RE: Backup MX records.
- From: <paul_lemonidis@xxxxxxxxxxx>
- To: "[ExchangeList]" <exchangelist@xxxxxxxxxxxxx>
- Date: Wed, 26 May 2004 17:13:41 +0100
Hi Al
The honest answer to your question is that I am not totally certain. I know it
does heavy modifications to SMTP traffic but having said that it only NAT's
SMTP traffic and modifies it in real time and does not run as Deamon in its own
right from what I understand. I can telnet directly from the Exchange server to
remote hosts. It is certainly not capable of storing and forwarding from what I
understand of it. Of course it may be acting as a mailer and just presnting the
remote host as a virtual session but somehow I would very much doubt that to be
honest.
I am sorry to be so vague but as you many have gathered I have little real
knowledge of how it works internally. From what we have discussed I am pretty
convinced the firewall is the problem.
Many thanks for your help.
Regards,
Paul Lemonidis.
----- Original Message -----
From: Mulnick, Al
To: [ExchangeList]
Sent: Wednesday, May 26, 2004 4:45 PM
Subject: [exchangelist] RE: Backup MX records.
http://www.MSExchange.org/
It *could* be a firewall issue. I assume from your post that your firewall
is in the mailstream as a mailer. If that's the case, then you're not failing
to send to the remote host, but rather to the firewall. It's a little
confusing from your post however. Are you using a mailer-daemon on the
firewall to filter traffic or does exchange have TCP 25 destination ability on
it's own?
Al
------------------------------------------------------------------------------
From: paul_lemonidis@xxxxxxxxxxx [mailto:paul_lemonidis@xxxxxxxxxxx]
Sent: Wednesday, May 26, 2004 11:35 AM
To: [ExchangeList]
Subject: [exchangelist] RE: Backup MX records.
http://www.MSExchange.org/
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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