This error usualy happens when one of our customers is trying to send mail through our SMTP (outgoing) mail server while connected to a network other then ours.  As a spam preventative we do not allow clinets to access our SMTP from outside our network without first authenticating.  You can setup authentication in your Outlook Express as follows<br>
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First you will need to open your Outlook Express.<br>
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This can be accomplished by clicking on the "Start" button, then going to "Programs" and clicking on "Outlook Express".<br>
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<img src="./start-oe.jpg"><br>
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Next you will click on "Tools" in the menue bar and click on "Accounts".<br>
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<img src="./tools-accounts.jpg"><br>
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Click on the "Mail" tab.<br>
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<img src="./accounts/mail.jpg"><br>
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Double click on your mailbox (by default it is named pop.nxs.net).<br>
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Click on the "Server" tab.<br>
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Click in the box to the left of "My server Requires Authentication".<br>
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You should now see a check mark in the box.<br>
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<img src="./accounts/serversauth.jpg"><br>
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Click on the button labeled "settings".<br>
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<img src="./accounts/servers-settings.jpg"><br>
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Make sure there is a dot in the circle to the left of "Use same settings as my incomming mail server".<br>
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If it does not have a dot in it click in the circle next to "Use same settings as my incomming mail server".<br>
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Click the button "OK" at the bottom.<br>
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<img src="./accounts/serversauth.jpg"><br>
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Click the button "OK" at the bottom.<br>
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<img src="./accounts/mail.jpg"><br>
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Click the button "Close" at the bottom.<br>
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At this point the problem should be resolved for this mailbox.  If you have other mailboxes that we are hosting then you will need to repeat this process for them.