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4.2.1 Unable to connect. "Attempted failover to alternate host ....

Here is one for those who like a challenge.


I have had a very interesting issue come up today.
I can find no reason for this what so ever.

I have seen a number of posts about this and most all of them are around port 25 for an ISP - this is not the case. ***

My servers have been up and running for about 3 years
their serevers have been up and runnig for about 2 years.
email has in the past been sent AND received with no trouble.
reciently my email will not leave the queue for thier domain.
My Users can send / receive email from any domain / user
their users can send / recive form any domain / user
***
The ONLY thing that is not working on both domains is the ability for us to send them emails.

I know I have not changed anything on my end.  My fellow minions claim to be guilt free.  And the receiveing end says no changes on his end.

- error message -
541 4.4.0 Primary target IP address responded with: "421 4.2.1 Unable to connect." Attempted failover to alternate host, but that did not succeed.  Either there are no alternate hosts, or delivery failed to all alternate hosts.


 - checked MX / DNS records with MxToolBox.com -
Mine cam back clean his has an issue with  
"Warning - reverse DNS does not match SMTP Banner" &
"Warning - Does not support TLS"
 
I can telnet to his server on port 25 and receive the proper responses from the exchange server ehlo command.

there are no 3rd party relay services.
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MPonto
Asked:
MPonto
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1 Solution
 
Simon Butler (Sembee)ConsultantCommented:
Can you actually do a full NSLOOKUP of the MX records from the Exchange server and get the correct results? Are you on the same ISP by any chance?

Simon.
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MPontoNetwrok AdminAuthor Commented:
Yes I get a full look up on both servers via nslookup
and
Yes we are both on the same ISP.
I have the isp looking into routing as well
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Simon Butler (Sembee)ConsultantCommented:
Being on the same ISP would tend to suggest that either the ISP is doing something, or the other side ahs their restrictions or network setup incorrect. I have seen some odd things happen where the subnet was configured incorrect on the router and local (ie same ISP) traffic was failing.

Simon.
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MPontoNetwrok AdminAuthor Commented:
It was in the DNS servers.  I was using a public server for forwarding requests (4.2.2.4) and when I changed it to the DNS or our ISP it works fine.
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