How do I resolve 'unable to verify account name or password' for BigPond in Apple Mail on Mac mini?

I was having trouble with sending emails from Apple Mail, Telstra advised me to delete the bigpond from the Mail accounts the re add it, I am unable to do this, it comes up with 'unable to verify account name or password'. does anyone have a work around for this problem please. Mac Mini, with OS 15.4.1.

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