When users sign up to a public instance and make their email their username, it becomes public accessible to all users. Users should be aware that this may publicise their email for spammers. Current sign-in text appears as:
"Your username will be used to log in to Confluence. This must be unique: for example John Smith could use jsmith. Alternatively you can use your email address as your username."
The public email risk could be emphasised by rewriting the login text similar to below:
"Your username is used to login. When choosing, select a username that is unique and includes your first name. Alternatively you can use your email address as your username, though this makes your email viewable publicly viewable."