You can set it up on the My Page of au Email Portability.
Please check below, for instructions.
Access the au Email Portability page on your smartphone.
Scroll down the screen.
Tap on "Click here for settings after application" [お申込み後の設定はこちら].
Enter your au ID and tap on "次へ" [Next].
Enter your password and tap on "Log In" [ログイン].
Tap on "Spam Filter Settings" [迷惑メールフィルターの設定].
The "Spam Filter Settings" [迷惑メールフィルター設定] screen will be displayed.
Set the various filters according to your preferences.
Has your problem been solved?