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:
firstname.lastname@example.org on 12/20/2011 1:55 PM
There was a SMTP communication problem with the recipient's email server. Please contact your system administrator.
<mail.xxx.com #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.