Thats the confusing bit
It wasnt from optus, it was a genuine nasty spam with links to a fake Optus site
"dear [blank name] Your account is overdue, click here "
plus a whole lot of incorrect details about my past months accounts
Happy to paste the text including the bad bits or send it to someone at optus to check out
You should have a option to look deeper into the "header" of the spam email.
Its possible for the email to look like its come from one address while actually been sent from another.