Thanks for your reply Kenobi, although it is very odd...
"As you mentioned POP3 creates local copies of emails and deletes the originals from the server"
No, I didn't mention that at all - all of the original emails DO remain on the server. I've just checked.
" the emails ( especially those in each devices sent folder) are tied to that specific device, and cannot be accessed via any webmail or any separate client on other computers."
So are you telling me that every email that we have sent from our individual email clients for the last 10 months (which I can still see in the live.com webmail Sent folder) have, in fact, not been stored at all and I'm making it up?
"I'd be interested to know how you were able to sync up the webmail Sent Folder via POP3."
Sure, I just entered the POP3 settings in each client: smtp.live.com, port 25; pop3live.com, port 995 when I set them up.
It's a bit worrying that I've had to explain that to the Technical Support that I'm asking for help from. It seems that this is a protocol that Telstra have failed or are unable to migrate, are unaware of, or are simply ignoring. Which is not really a solution that I was asking for.