If iCloud Mail isn't working - Apple Support
https://support.apple.com/en-us/HT20352814.05.2021 · Click Accounts, then select your iCloud account in the sidebar. Under Account Information, make sure that the account is turned on and the status is online. On your iPhone, iPad, or iPod touch, go to Settings > [your name] > iCloud and check that Mail is turned on. Make sure you that haven't exceeded your iCloud storage limit.