Solution I found several years ago was to set Google and/or Yahoo to collect and send our Bigpond emails.
Gradually worked through all those on our mail list to make sure they changed their addresses to our Google&/Yahoo.
Problem once, when passwords reset self forgot to change password on Googe/Yahoo, was quickly fixed.
Push for everyone to use MS-W continues, while for self change to linux fixed 90% my problems, so whatever happens with next Telstra/Bigpond email changes, they unlikely to be problem.