Hmm, webmail doe not use mail.bigpond.com, never did. It is years since I abandoned Telstra email, but I don't think anything uses those server name any longer. I will just google that assumption. Perhaps try using the settings provided by Telstra here https://www.telstra.com.au/support/email/imap-pop-smtp-mail-server-settings. mail.bigpond.com is most certainly wrong. Note account settings can be found by right clicking the account in the folder pane. You are not adding a new account here.