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Try this one,
Tap Settings > [your name] > iCloud & turn off Mail on your iPhone. Also tap Settings > Mail > Accounts > Fetch New Data, and turn off Push.
You did not reveal your iPhone model. For some versions/ model, you can try like,
Open Settings app on your iDevice > iCloud . Here scroll down under all the settings to find "Sign Out" & tap to turn off.
But it is recommended that, do not delete the account.
Here is a help from Apple,
Here is a help from Apple, https://support.apple.com/en-in/HT207689