If you are using the au Email Portability service, you can reconfigure the email app from My Page.
Please see below for detailed instructions.
Tap on "Email app settings" [メールアプリの設定].
iPhone/iPad: Tap on "iPhone/iPad Default Email App" [iPhone/iPad 標準メールアプリ] (proceed here and check STEP 3: How to Set Up the Email App).
Android: Tap on "Android au Email App" [Android auメールアプリ] (proceed here and check STEP 3: How to Set Up the au Email App).
Gmail: Tap on "Check the ID and Password Required for Email Settings" [メール設定に必要なID・パスワードを確認する] (proceed here and check STEP 3: How to Set Up the Gmail App)
For example: If you cancelled on April 1st, 2021
au ID before cancellation: 080XXXXXXXX
au ID after cancellation: 080XXXXXXXX_20210401
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