Error when sending to a specific email address

We are receiving NDRs when we try to email one specific email address, does not matter if we send the email from outlook or through our web application.

We receive the following message back: on 12/20/2011 1:55 PM
            There was a SMTP communication problem with the recipient's email server.  Please contact your system administrator.
            < #5.5.0 smtp;553 sorry, that domain isn't in my list of allowed rcpthosts (#5.5.3 - chkuser)>

I masked the mail name and the email in question.

We use Exchange Version: 6.5.7638.1

I tracked the message through messaging tracking center and it seems that the message is not going out of our server.

Any ideas?
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AmitIT ArchitectCommented:
Check the disk space.
htabbachAuthor Commented:
Thanks for the response, if you are referring to the disk space on the Exchange server, I still have 100GB available so this is not the issue.
Kamalasekar ParthasarathyMessaging SupportCommented:

Go to mxtoolbox site and check your domain is in blocked list or not..

Link :

Also, please check the below link and perform the internal and external test..

It will show the exact error message and u can fix it..

Also, download the tool called SMTPdag and send a test mail to the destination domain by using this tool.. you can also fix by using this.

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