Found a queued message seems affected by the recipient mail server, why?

This is using MS Exchange 2010 Server. In queue viewer, found a queued message with the following error messages:

451 4.4.0 Primary target IP address responded with "554 5.7.1 delivery not authorized." Attempted failover to alternative host, but that did not succeed. Either there are not alternative path.

Please help.
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Simon Butler (Sembee)ConsultantCommented:
The remote side is rejecting or greylisting your email.
If the latter, it should be delivered later. If it fails, then you will need to find out from the other side why they rejected your message.


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MichaelBalackAuthor Commented:
Hi all,

I did a check on my mail server using dnsgoodies > reverseDNS, and found that the ptr return is not what was registered. This is because the ISP the provided the Internet line for this mail server has to explicitly grants this mail server to work, in order to stop spams.

After request and granted approval from ISP,  no more queued mails.
MichaelBalackAuthor Commented:
The problem was resolved. It was because the MX IP has yet to grant approval from the ISP, and it works perfectly after getting the approval. Sembee pointed out the main root course - mail rejected because this mail server is still falls short of some essential requirements.

Thanks Sembee.
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