3170 error on Macbook

I offer hosting to customers and a customer has recently got an error when moving their email to ourselves. I don't think its web/email hosting related as the email account works perfectly on our PC's & MACs just not on this one customers Macbook.

The error they get is:

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Error - Outlook cannot find the server. Verify the server information is entered correctly in the account settings, and that your DNS settings in the network pane of system preferences are correct.

Details - Outlook cannot find the server. Verify the server information is entered correctly in the account settings, and that your DNS settings in the network pane of system prefferences are correct.

Account Name "XXXXXX"

Error Code: -3170

Any one have any ideas why this is happening? I'm no MAC specialist so haven't been of too much use to date. But would love to get it sorted. I've done remote support etc. to verify they were entering the correct information - and it's all correct. So with the correct details on the account (on their MAC) they get this error.

They're using Outlook.

Regards and thanks in advance! :)

Charlie S