Have you considered using a 3rd Party email account rather than an ISP supplied email account?
The big advantage is if you change ISP you do not have to worry about your email.
It sounds like your D.O.B. may have been entered incorrectly into the Optus system.
You've posted in the Optus Sport forum, are you trying to change your e-mail for your login?
Please shoot the Optus Sport team a message for assistance.