If you are using the au Email Portability service, you can reconfigure from My Page.
Please refer to the steps below.
Access My Page and log in with your au ID and password.
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)
Android: Tap on "Android au Email app" [Android auメールアプリ]. (proceed here) (in Japanese only)
Gmail: Tap on "Check ID and password required for mail settings" [メール設定に必要なID・パスワードを確認する] (proceed here). (in Japanese only)
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