I´m guetting an error 421 5.5.0 whem sending messages from an exchange2007

I´m guetting an error 421 5.5.0 whem sending messages  from an exchange2007 to a exchange 2003 behind a ISA2004, In the ex2003 side i´m able to receive mail from others exchange´s 2007 , any clue?
rss_inforAsked:
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JoWickermanCommented:
Hi rss_infor,

Have a look at this post:

http://www.experts-exchange.com/Software/Server_Software/Email_Servers/Exchange/Q_21382224.html

Let me know if helps.

Cheers.
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SubsunCommented:
Can you paste the full NDR message?
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rss_inforAuthor Commented:
Because I had to change some DNS configurations
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