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Exchange outgoing email blocked

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Exchange outgoing email blocked - 19.Apr.2015 2:52:07 AM   


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Joined: 19.Apr.2015
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We are running SBS 2003 with Exchange Server 2003. The purpose of this question is more of an understanding how email works with exchange server. The problem started about two weeks ago when we switched ISP to Comcast. At that moment email was being blocked by Hotmail and Gmail. We would get the following message back.

There was a SMTP communication problem with the recipient's email server. Please contact your system administrator.
<nhnr.org #5.5.0 smtp;550 OU-002 (COL004-MC4F51) Unfortunately, messages from 73.189.0.xxx weren't sent. Please contact your Internet service provider since part of their network is on our block list. You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors.>

MXTOOLBOX says that the public ip address comes on one blacklist - Spamhaus ZEN.

Here is the question. If I send mail through exchange to a hotmail account it will be bounced back and undeliverable with the above message. If I configure a POP account and send an email to hotmail it will successfully go through. Why does the POP work and not the Exchange?
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