Exchange 2010 DnsConnectorDelivery

I'm having an issue with one domain. Mail is not being delivered to them from our server. Every test message sits in teh Queue with the "451 Primary target ip address responded with: 421 4.2.1 Unable to connect. Attempted failover to alternate host., but that did not suceed. Either there are no alternate hosts, or delivery failed to all alternate hosts."

I've tested the domain and done everything I can think of to get it to work. It's jmfinancial.net. I've seen a delay on the SMTP test, but nothing concrete. It's driving me nuts and of course it's a friend of our CEO. He's blaming our system because he can "get email fine from other people". I'm almost positive it's on his end, but I have no proof and I'm out of ideas. We can send and receive email fine to all other domains. We're not blacklisted and all of my Domain/MX checks return everything ok on our end. Any help would be greatly appreciated! We're running exchange 2010.
jmchristyAsked:
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jfletchsterConnect With a Mentor IT ManagerCommented:
On your server can you telnet the destination server DNS name and IP address and does your server resolve the same IP address as external sources such as MX Toolbox. AS your DNS might be giving incorrect information, so your trying to deliver to the wrong server.
Also see if Telnet works, to make sure there is no issue with traffic ie.e firewall beween your server and theirs.
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Sanjay SantokiConnect With a Mentor Commented:
Hi,

I would like you to try sending emails through command line to see where the issue is.

telnet MXofremotemailserver 25
helo yourdomain.com
MAIL FROM: <user@yourdomain.com>
RCPT TO: <user@remotedomain.com>
data
FROM: <user@yourdomain.com>
test
.
quite

Thanks,
Sanjay Santoki
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jmchristyAuthor Commented:
My server does resolve the IP address correctly, but I cannot telnet to it from the mailserver. I just says connection failed. I am however able to telnet to other mailserver from my exchange server. I just tested it with one of our customers. So, telnet is working, just not to their server.
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Sanjay SantokiCommented:
Hi,

Furthermore, please be sure you have reverseDNS configured for your relay IP. That needs to be configured from ISP.

Thanks,
Sanjay Santoki
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jmchristyAuthor Commented:
I do have rDNS configured. It matches my SMTP banner as well. I'm not sure why it's not opening the connection to his mail server. I'm definitely able to contact other mail servers from my exchange box. His appears to be the only one that doesn't connect at all. I have an external XP machine that I am able to telnet into his mail server from. So, it's as though only my mailserver telnet is being dropped.
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jfletchsterIT ManagerCommented:
Do you have multiple external static IP's? Some routers/firewalls use all the external IP's as part of thier NAT pool, which can cause issues with sending email, as the router picks a different external IP at random for each transaction, causing the rDNS to fail. The way to check this it to get the server to do an External IP look up to a website a few time with a fresh browser each time and wait a few min in between each try.
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jmchristyAuthor Commented:
Update: It does appear to be something on their end. The owner isn't sure what it could be, and I'm not going to troubleshoot their issue. Thank you for the help!
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