Judging by a number of our customers negative feedback, we get the feeling that telstra Tech Support people don't really understand Apple Mail (mac Mail)
When setting up POP bigpond mail accounts on a new Mac using Apple mail, the mail can appear as “offline” with the result that mail cannot be sent and backs up.
Checkout this video from Telstra and see if it works for you.
When you configure a Bigpond POP mail account the auto-complete does not always configure the SMTP settings correctly. The outgoing mail server (SMTP) may show as “Bigpond Offline”
If it doesn't, check the following in Apple Mail
mail > preferences > accounts
un-tick the “Automatically manage connection settings”
and try ticking the box “Use TLS/SSL” and set authentication to “password”
This fix has worked for a number of clients of The IT Guys (WA) we hope it works for you.