Error message when send email

When I send an email to certain email address I get the following error message in  a reply message...

There was a SMTP communication problem with the recipient's email server. Please contact your system administrator.
  <Server #5.5.0 smtp;553-mail rejected because your IP is in DUL. See>

What is the problem here?

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dwelldonAsked:
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NiclasHCommented:
BarthaxCommented:
> your IP is in DUL

The reverse DNS of your server shows that your server is "in DUL" - "in Dial-Up Land".  Most likely you are using a DSL connection or some other form of connection which could be construed as a home-user-access-dial-up equivalent.

To get around this problem you could add another SMTP Connector which uses the address space of the rejecting domains & forwards the e-mails to your own ISPs SMTP relay servers instead.  Alternatively, you could send all your e-mail through your own ISP's relay servers.
dwelldonAuthor Commented:
I am going through a cable modem, not sure about setting up SMTP connectors have you any instructions?
BarthaxCommented:
Right-click your Connectors folder in the Exchange System Manager and choose New->SMTP Connector.  On the General tab, enter the relay server at your ISP in the Forward [...] box (check the radio dial to enable it).  Under the Address Space tab, add in an SMTP type for each of the destinations required with a cost of 1.  Click OK when done & edit the pre-existing SMTP Connector & on it's Address Space tab, edit the * entry and change the cost to 25.  The new connector will then take precedence over the old one but will only deliver mail destined for those specific locations.

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