Are you trying to do it through Outlook.com or Outlook (the application)?
You have probably been migrated over to the new Telstra Mail. The settings are on this page
https://www.telstra.com.au/support/category/email/telstra-mail/set-up-telstra-email-on-outlook-2013
or for Webmail, you should log in through https://email.telstra.com
