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Ok in order to send and receive mail or synchronise additional items like calendar and contacts your device needs to authenticate successfully with your mail server. If authentication has failed then it could be due to a mismatched stored username and password ***** a corruption with the mail profile stored on your device.
Please can you remove and re-add the account to your device to get this working again.
Follow the steps here on your device to remove the account:
Once removed then follow the steps here to re-add the account to the device - https://support.apple.com/en-us/HT201320
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