Thanks for that info and choosing the Apple Support Communities. If I understand correctly, you are unable to change the default account used in the Mail app on your iPhone. To help resolve this issue, first go to Settings > Accounts & Passwords and make sure multiple accounts have Mail syncing turned on. Then go to Settings > Mail, scroll down to the Composing section, and tap the option for Default Account and set the account you prefer.
Thanks for that info and choosing the Apple Support Communities. If I understand correctly, you are unable to change the default account used in the Mail app on your iPhone. To help resolve this issue, first go to Settings > Accounts & Passwords and make sure multiple accounts have Mail syncing turned on. Then go to Settings > Mail, scroll down to the Composing section, and tap the option for Default Account and set the account you prefer.
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How to change default email on IPhone X iOS 11.2
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