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Email bouncing back - DNS error message

Hi there this is my first post. I have Exchange 2000 running on a Windows 2000 server. A couple of weeks ago a I found out certain emails were getting bounced back to users with the following NDR:

The following recipient(s) could not be reached:

      Test User ( on 26/11/2004 15:49
            There was a SMTP communication problem with the recipient's email server.  Please contact your system administrator.

            <ourserver #5.5.0 smtp;551 <>: Sender address rejected: sender DNS has bad configuration>

Does anyone have any ideas what is the problem as I can't find out anything relating to this error. I have checked all the blacklists and we are not on any and there are no DNS errors at our end.

Any ideas would be much appreciated.
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Chris Dent
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Oops, I should have read that a bit more. Your own server is generating the errors?

Any event log errors at all associated with the bounce back?
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Thanks for the advice. I have tested with those DNS tools and it appears to me to be a reverse DNS problem at the ISP's end. Having confronted them with this they have said that they have known about it we are not the only one with this problem. We shall see tomorrow after their work. I shall post an update to this when I get further news.

Thanks again for the help.

No problem, hope that fixes it for you.