Email sent from an exchange 2007 to a specific address stuck in a queue

Dear Experts

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http://www.experts-exchange.com/Soft..._22634886.html

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I have got a client she has an exchange server 2007 installed on SBS 2008.

She has no problem to send and receive emails through the exchange server except when she sends an email to this address ([email protected] ,I made it up)!!!!!!

The message stays in the exchange’s queue and it won’t be delivered to the nominated address, with the below error :

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451 4.4.0 Primary Target IP address responded 421 4.2.1 unable to connect.

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I couldn’t test a different email address on this domain (abc.com.ih), because I don’t have other than elizabeth

If her email on the exchange server has a problem it shouldn't be able to send emails to all domains from the first place

I tried to flush DNS on the client’s exchange server but no joy

What I did as a troubleshoot I sent an email to : ([email protected]) from our work exchange server 2007 installed on SBS 2008 (i.e. different exchange but almost has same setting as the client) the email went through and the recipient replied me to confirm that he received my email.

Regards

I had my guess but I Googled this anyway. Kinda confirms what I think. My feeling is that either the DNS for the sender's server is missing/corrupt/wrong, or the MX records for the recipient are not correct. There is also the possibility that something at the sender is affecting outbound mail for the recipient's destination (antivirus, firewall, mail filtering program, etc.)