Email fails to specific domain with '421 4.2.1 Unable to connect'

We have a simple Exchange 2010 single server setup. Everything has been running fine for years. We have one domain that we are attempting to send an email to that fails. All other emails are flowing fine. The error in the queue is:

451 4.4.0 Primary target IP address responded with: '421 4.2.1 Unable to connect' Attempred failover to alternate host, but did not succeed. Either there are no alternate host, or delivery failed to alternate hosts.

The last successful message to this domain was about 8 weeks ago. I can do an nslookup on the MX records and see them there. I have done an mxtoolbox.com backlist check on my domain and that is coming back fine.

Ideas?

Thanks
GlennCameronAsked:
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Dave BaldwinFixer of ProblemsCommented:
Sounds to me like that server is not working or available anymore.  The fact that it is still listed in DNS does not mean that it is available.  You would have to find an alternate method of contacting them to find out what is happening.
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Dave BaldwinFixer of ProblemsCommented:
The usual thing is to 'telnet' to their mailserver and see if it responds.  Or if there is a contact email address, you could try to send them an email thru Yahoo or Gmail to see if they're still there.
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GlennCameronAuthor Commented:
I cannot telnet to the mail server. I get 'Connecting to xxxx' then 'Connect failed'.
FYI the domain is a US state government department so they're still there :-)
Their IT is saying that's its an issue on our server.

Thanks
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lindentoCommented:
Hi,

check that your DNS is resolving the MX records correctly, easy check for this is do an mxtoolbox.com mx check.

If you try to telnet their listed mx on port 25 and can't reach it,
you need to rule out firewall on your end blocking access to their IP.
Also try telnetting to that mailserver from outside your company network(effectifly testing if firewalls are limiting you)

Best Regards
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GlennCameronAuthor Commented:
I have done the mxtoolbox.com mx check and they appear to be correct. Was going to try the telnet from home tonight.

Thanks
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GlennCameronAuthor Commented:
I cannot telnet to the server either inside my corporate network or at home. Two completely different ISPs and of course no firewall at home.
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GlennCameronAuthor Commented:
The destination company finally admitted that they were blocking the traffic.
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Dave BaldwinFixer of ProblemsCommented:
Interesting.  Did they say why?
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GlennCameronAuthor Commented:
Spam attack from somewhere in China so they have put a lot more restrictions in place. You would of thought that they could have told us that when we first raised the issue!
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