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no valid mail exchangers

Posted on 2014-03-06
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Last Modified: 2014-03-12
Hi All,
We have a rather weird problem where several customers are periodically unable to send to our domain.  The problem is very hard to replicate, and I'm 99% sure the issue is on their end, but I'd like to pointpoint the issue, and need some ideas on where to look.  

The NDR they receive is:

"This message was created automatically by mail delivery software.
 
A message that you sent could not be delivered to one or more of
its recipients. This is a permanent error. The following address
failed:
 
domain has no validd mail exchangers"
 
However, as I said, our MX records pass all the SMTP tests, and most users have 0 problems.  When testing their MX records, I get Reverse DNS mismatch ("Warning - Reverse DNS does not match SMTP Banner").  Could this be related to the issue?

Any other ideas on where to poke around would be helpful.  thanks!
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Question by:cuiinc
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by:Lee Ingalls
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"Warning - Reverse DNS does not match SMTP Banner" will not stop from sending email.
I have a full email testing toolbox if you would care to share the domain name.

Have there been any DNS MX record changes?
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Expert Comment

by:Lee Ingalls
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A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address failed:  domain has no validd mail exchangers

If there were multiple recipients and you were replying all to their message... I've seen where the sender included an internal email address that when you reply to all cannot reach from external.
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by:cuiinc
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No DNS changes on our end whatsoever.  Our domain name is cuiglobal.com.  Like I said, I haven't found any testing issues with it thus far...

I don't quite understand your 2nd comment about the Reply All.  Would you mind expanding a bit on that?  Thanks!
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by:Lee Ingalls
Lee Ingalls earned 250 total points
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I'm constantly have to demonstrate to our customers that the issues does not lay on our end.

When I read that "A message that you sent could not be delivered to one or more of its recipients." I was curious if you were replying to a message your customer sent that had multiple recipients. If your user replied to all... that perhaps one of the original recipients was a an internal email address on their system. I see it all the time from our large customers that have internal and external email relays.
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davorin earned 250 total points
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"Warning - Reverse DNS does not match SMTP Banner" will not stop them from sending email, but it can stop your server from accepting mails from servers with this problem. Or it can mark mails from that source more likely to be spam.

On exchange 2013 reverse DNS check is done by sender reputation agent.
http://technet.microsoft.com/en-us/library/bb124512.aspx

To troubleshoot this issue I would focus on the single mail and I would try to find any evidence of it on your mail server logs. Or they could also be rejected by your firewall. Look at that logs too.

If you find some evidence of connection trying do deliver email, most likely the "problem" is on your side. That could mean that your antispam filtering is rejecting their mails because of wrong rDNS.
If there is no evidence at all, then the problem is on their side or somewhere in between.
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