POP connector failing with errors in Exchange 2007

Set up an Exchange 2007 server which initially was receiving external email ok via the inbuilt MS pop connector.  Then suddenly it started to fail. Each time the pop connector goes to retrieve mail it fails to do so and I see an Event Viewer entry for each recipient who has mail waiting to collect:
"An error occurred while delivering mail to '<email address>' on the SMTP server 'localhost'. The error code was 0x800ccc69. Verify that the mailbox exists and that the Exchange receive connectors are configured properly."
A google search has only found me 2 useful hits: Low disk space (Ruled out as I have 50Gb)
and a suggestion to set up an internal relay on the Hub Transport under Organization in the Exchange MMC. But I can't see a mention of Relay on any of the tabs within Hub Transport.
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Jeffrey Kane - TechSoEasyConnect With a Mentor Principal ConsultantCommented:
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