Unable to send from Exchange 2010 to Exchange 2003 Coexistence mode

I get the error:

 "451.4.4.0 Primary target IP address responded with 421.4.4.2 Connection dropped due to SocketError"

I am able to send from 2003 to 2010, and from 2010 to internet email addresses and back again however the only route that doesnt work is from 2010 to 2003.

I have created a bidirectional RGC but still the same error appears. Googling this issue only leads to dead ends.

I would massively appreciate it if anyone could help!
Sean GaughanIT administratorAsked:
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Sean GaughanIT administratorAuthor Commented:
Thanks Amit, I didn't see anything that rings true in those posts though. Why would it work ok from 2003 to 2010 and from 2010 to and from the internet?

Surely the ports would be open for it to do that?

Thanks,

Seán
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AmitIT ArchitectCommented:
I remember this one. Can you check 2003 Default Virtual Server setting. Just read this article.
http://demazter.wordpress.com/2010/04/08/exchange-server-mailflow-during-migration/
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Sean GaughanIT administratorAuthor Commented:
Ah ok. So should I created a new SMTP connector in 2003 with the purpose of routing to 2010 Exchange?

Sorry if I sound a bit dense, its my first time doing this.
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AmitIT ArchitectCommented:
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Sean GaughanIT administratorAuthor Commented:
Already done that unfortunately. :( Still wont play ball. I don't get it how they both see each other with connectivity as follows working fine:

Exchange2003>Exchange2010
Exchange2003>Internet
Internet>Exchange2003
Exchange2010>Internet
Internet>Exchange2010

With the only route not working being:
Exchange2010>Exchange2003.

Its so weird. :(
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AmitIT ArchitectCommented:
Are you able to telnet from 2010 to 2003 server on port 25?
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Sean GaughanIT administratorAuthor Commented:
Hi,

No it failed, I got to the bottom on it. The IP of the Exchange2010 was not on an allowed list in the default SMTP server for exchange 2003 server!

in Exchange manager> Exch2003>Default SMTP Virtual Server > under the Access  < Selected "Connection" and "Relay" and allowed on both.

You really helped get on on the right path there! I really really appreciate it! :)

Seán
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AmitIT ArchitectCommented:
Great thanks :)
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Sean GaughanIT administratorAuthor Commented:
This was the eventual fix.
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