Unknown person using my email address

Did you revealed credentials to access iCloud account , like password and if account is set up with secondary authentication in which three answers for security questions , rescue mail address and its password , date of birth , credit card details ( optional ) to another person .

Or the account is set up with two factor authentication , is your iPhone in possession with that person to get codes .

How do you knew that your account is compromised , in any case if you revealed the credentials change all of them see the article If you think your Apple ID has been compromised - Apple Support

The email address can be known to other person , but to use that Apple ID if other account credentials are not known he can't access the www.icloud.com sending mails will be possible only when he has control over the account .

If you wish erase permanently the Apple ID address / account from apple server , consult apple support senior advisor and follow the article How to delete your Apple ID account - Apple Support

Write down the alfa numeric code , and don't sign in with Apple ID or send mails from it up to 30 days , as erasing will start .