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SMTP error 451 4.4.0 SMTPSEND supiciousremoteservererror from Exchange Online Protection

Using Exchange server 2003 on premises server connected to Exchange online protection server for spam filtering. Sometimes when receiving email from the Exchange online Protection I get SMTP error 451 4.4.0 SMTPSEND supiciousremoteservererror remote server disconnected abruptly on the Exchange online Protection server.  The exchange server will receive the email and process it to the user. The problem is the Exchange online protection does not think it sent the email so it sets the email as pending and will try to send it again several times. This causes the user to get duplicates of the email. I have opened a case with Microsoft and they said everything was correct with the exchange online protection server. Any one ran into this problem? It is on all cisco network and I have checked that smtpfixup is disabled.
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Darrell_Milam
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Darrell_Milam
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Simon Butler (Sembee)ConsultantCommented:
That NDR is not an Exchange NDR.
Therefore something else is causing it. Do you have any other third party tools on the server or in the router that could be causing it?

Simon.
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Darrell_MilamAuthor Commented:
There is No third party tools on the server just running the latest version of Exchange 2003 on 2003 server.  I do not manage the router or network. I can only manage the server but have had the people that manage it check to make sure no filtering for smtp is on.
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Simon Butler (Sembee)ConsultantCommented:
The NDR is an Office365 NDR I have just discovered, but it is referring to a problem with the remote server.
Exchange Online Protection is part of Office365.

No easy fix unfortunately, a managed solution like EOP means you have no options with regards to configuration. The only thing you can do is get whoever manages the network hardware to check again that everything is disabled for SMTP filtering.

Simon.
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Darrell_MilamAuthor Commented:
I had them check and still getting error. I can telnet to port 25 with no problems.
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Simon Butler (Sembee)ConsultantCommented:
A telnet test isn't really going to prove anything, as you cannot telnet in from the EOP network.
There isn't much more I can do to assist you. You are stuck between two managed services. EOP will do nothing because they will ay it works for everyone else, and your network management company are claiming that everything is working fine - when it obviously is not.

Simon.
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