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Exchange error 554 <mail.mydomain.com[xx.xx.xx.xx]>: Client host rejected: Access denied

We are getting this error while sending emails to some external clients:
Exchange error 554 <mail.mydomain.com[xx.xx.xx.xx]>: Client host rejected: Access denied
I asked the people responsible for the external clients emaildomain, but that seems to be all fine.
So it looks like our exchange is sending out this error.

What could be the problem here??

mx record is fine, ptr is fine
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1 Solution
did you do a complete domain check on www.dnsstuff.com?

it maybe that you have been blacklisted by a spam database, you should check that out too.
Error 554 indicates that  your external client's mail server rejects your emails. This seems to be true also because it is happening only with a particular destination.
Do you have the smtp logging activated? Did you look into the logs?
suppsawsAuthor Commented:
this domain WAS blacklisted, but it isn't anymore.
the domain is @qatinvestments.com
If I look on dnsstuff everything seems fine ....

One  more thing, there was a NDR attack to that domain, do I did dome changes on exchange to get rif of those attacks. Btw, the server is NO open relay.
SMTP logging was on, but I kept on getting thousands of errors that the server couldn't relay (because of the ndr attack)
If this is an error of the destination server it could be some dns settings on my domain that are wrong?
suppsawsAuthor Commented:

when I do the test on mxtoolbox.com I'me getting this warning:
Transaction Time: 11.266 seconds - Not good!
What can I do about this?
suppsawsAuthor Commented:
very strange behaviour. First the reverse dns was ok, 10 seconds after that it was:
 Rev DNS Check: Reverse DNS FAILED! This is a problem.

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