For 24/7 support message our team of experts via the My Optus app.
Keep getting the message
cannot get mail username or password incorrect
this started suddenly after having worked well for a very long time
Are you seeing the prompt on all devices?
Are you able to access the Optus Webmail portal at all? If so, you'd simply need to remove the account from your device(s) and set them up again.
If you're still seeing the prompt, send the team a message and we'll send out a temporary password.
You'll then need to follow the instructions to change it to a password of your choosing.