Username and password
E-mail correspondence usually contains information of a private nature. Access to messages on the mail server is therefore only possible after the user has logged in using a username and password.
Login data is determined by the user himself, but not always. For example, at work, the login name is assigned by the employer so that the e-mail addresses of the employees have a uniform form. The password should be strong (with a combination of uppercase and lowercase letters, numbers and special characters) and should not have a specific meaning, especially not predictable from commonly available information about the user. So your partner's name, date of birth and favorite sport are not the best choice to secure your email.