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Exchange sending retry and MX records

Posted on 2004-09-02
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Last Modified: 2009-07-29
This is a strange one so hang in there with me on the explaination:

We are an Exchange 2000 shop and our DNS and network is provided by the larger organization using BIND DNS.  

Here is how our MX records are set up (the names have been change to protect the innocent):
our.domain.com  MX preference = 10, mail exhanger = mailserver1.our.domain.com
our.domain.com  MX preference = 20, mail exhanger = mx.domain.com

Meaning that if the server mailserver.our.domain.com were not available the mx.domain.com server would receive the email and queue it up.  

This main organization does not allow any communication with domain servers over port 25.  Meaning that the mailserver1.our.domain.com is not accessible by the outside world and all mail should be routed through mx.domain.com.

There is an external group trying to send us e-mail (they are an Exchange 2000 shop as well) and the email does not get delivered.  After about 2 days they receive a message saying that the mailserver1.our.domain.com server was unavailable.  Their logs confirm that the server was unreachable.  However, it does not seem to me that their Exchange server is attempting to send through the second MX preference mail exchanger.

Is there a setting in Exchange that could account for this?  Any ideas?

Thanks,
Dan
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Question by:dllewis
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by:Sembee
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There are no settings in Exchange that can make it use the second MX record other than what it works out itself from the SMTP communication.

This is really a problem for the other end to look at - especially if it is just one other site with the problem. They should do a telnet test to port 25 of the primary MX records and see what response comes back. There are certain SMTP responses that can be returned that make an SMTP server (beit Exchange, Sendmail or whatever) that make it give up on the first MX and try the second.

It could also be stale DNS information. I presume you have asked them to flush their DNS cache?

Simon.
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by:dllewis
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Thanks for the response.

I figured that unless the e-mail server just doesn't follow the RFC what could it be.

If the other end tries to telnet to the primary MX they would get denied because of the firewall.  If they try to connect to the secondary it does work and a ehlo get a normal response.  Delivering to the secondary is where the issue seems to lie.  That their e-mail server isn't trying the second.  Or so it seems.

We have checked and rechecked the DNS, flush - yes.  Still the same issue.

Thanks,
Dan
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kristinaw earned 250 total points
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dllewis,

i've run into this same issue with two other orgs recently. we have had our primary mx down periodically lately for testing various spam filter and smtp servers packages. since we have a secondary smtp server with all the appropriate mx records in dns, i wasn't worried about having the primary down. however, i ran into the same issue where the other orgs mail server never seemed to try the seondary mx, and a couple of days later they get the unavailable message. I was able to work with an admin at one of the affected sites who patched his product and got the issue resolved. i wasn't as successful with the other org, and wound up swapping mx preferences for the time being which immediately fixed the issue for that org, his mail came right through after the swap. however, neither org was running exchange on their end, but it still might be worth asking them if they have all the latest fixes applied.

kris.
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