Hi All - after changing the IP address from Dynamic to Static, mail.bigpond.com was showing "Connection Refused" on the Linux Sendmail server.
Telstra offered to change the fixed IP again to see if this fixed the issue.
It didn't fix the issue with a second static IP address and while I was on the dynamic IP for a day SMTP outbound e-mail via mail.bigpond.com did work - so this issue is something with their fixed IP address allocation and possibly trusting 'relay' through it.
I have repeatedly told them using mail.bigpond.com is the same for any SMTP outbound, regardless of whether I have a Linux e-mail server or a Microsoft Exchange server.
Solution is to ensure your DNS (Reverse DNS and spf) is setup and then bypass mail.bigpond.com simply by delivering directly via port 25 to the Internet.
My actual solution now is to use my other Internet connection for outbound which will go through tpg.
Also to note, the second static IP was also listed by SpamHaus but now I review this I believe this is more a SpamHaus thing locking down anything they perceive as a dynamcally assigned range.