After many hours of searching, reading, calling and chatting (the last two over repeated calls to Telstra BigPond techies), it seems that in about 2012, Telstra started migrating old email account users to Exchange or something, then in about 2014 migrations continued on - but this time to Outlook.com accounts. These migrations were apparently to keep BigPond emails up with the times and in the process changed to IMAP.
Mine was not migrated either time and is still the same old BigPond email account.
Now Telstra informs me that migrations have ceased... More hours of research & helpline calls ensued.
Long story short - the biggest provider in Australia (Telstra) has left some of their long-standing customers behind, and we now cannot use some of the most widely used software on the planet (MS Office Outlook App for iOS devices) to access our BigPond emails. MIcrosoft has seen fit to exclude archaic POP and POP3 email accounts from being added - fine if we were migrated to either IMAP, Outlook or Exchange. Otherwise a no - go!
I will be absolutely thrilled if someone can advise any different info, or a usable workaround that doesn't involve having to change my actual email address ... please.