Exchange error sending to certain domains "421 4.2.1 unable to connect"

I am not an exchange guru but I know enough to be dangerous.
I havs an SBS2011 with Exchange 2010 for my business (5 users). Mail is working for the most part however there are a few domains that we can not send mail to. The messages sit in the queue until they finaly fail. The error we are getting is:

451 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.

95% of our messages get delivered just fine, with the exception of a few domains where we get the above error. We are not on any SPAM blacklists. I don't know where to begin.

Please help.
wilkox1Asked:
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PostmasterCommented:
Try an MX record lookup (http://mxtoolbox.com/SuperTool.aspx#) of the destination SMTP domain.
This will show you the public IP for that mail server - make sure your system is talking to the same IP that is shown.

It may be that you have an entry in your own DNS for that domain - if so remove it to force the lookup to go external and get the right IP.

If the IP looks OK - then you may need to contact the IT support of the company and seek an advice.
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Md. MojahidCommented:
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