Exchange07 451.4.4.0 Error

I am having an issue with sending message to specific domains and getting a 451.4.4.0 error.  The messages come back to the users as Delivery Delayed in their Outlook and I see the 451.4.4.0 in the Queue as well as the undelivered messages.  I have examined the MX records and verified using online tools such as MX Lookup tool.  I have turned off our AV and tried it with no luck.  When I telnet 25 to the SMTP servers in question I get no response from them.  The only thing that has changed is we upgraded from SBS2003 to SBS2008.  Our inbound mail is filtered by a 3rd party spam tool but all outbound mail goes direct from us.

Any Ideas.

Thanks,

Kevin
kbtobeyAsked:
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kbtobeyAuthor Commented:
After some further testing, we determined that the issue only happens on Windows 2008 servers.  Any box running Server 2003 was able to telnet 25 to any of our problem mail server just fine.  With that new piece of information we found the following Microsoft Article:

http://support.microsoft.com/kb/951291

Apply the fix resolved the issue on our test 2008 server and we will apply to the production server tonight because a reboot is required.

Thanks

Kevin
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Rajith EnchiparambilOffice 365 & Exchange ArchitectCommented:
If you can't telnet to the other end server, probably it is down.

Put the mx record of the domain in question in http://www.mxtoolbox.com/diagnostic.aspx and see whether it connects.

If all of your emails to other domains are going fine, there isn't much you can do. The problem lies in the recipient end.
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Rajith EnchiparambilOffice 365 & Exchange ArchitectCommented:
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Rajith EnchiparambilOffice 365 & Exchange ArchitectCommented:
Good to know.
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kbtobeyAuthor Commented:
Resolved issue by disabling Network Autotuning.
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