This error indicates that TaskCentre cannot communicate with the SMTP server.
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To resolve this issue, update the SMTP settings:
Open Task Manager
Click Manage, Tools, Output, Send Message (SMTP)
On the Main tab, ensure that the server address is correct and the correct authentication method is enabled (or not enabled if using no authentication).
Ensure that if authentication is in use, the correct credentials are being used.
On the Advanced tab, ensure that the correct port number is being used.
Click OK.
Assuming all credentials are correct then the issue will be with the SMTP server itself and your own IT team will need to investigate further.