```
export interface User {
  email: string;
  active?: boolean; // Optional property
}

/** The lowercased addresses of the active users, in order. */
export function activeEmails(users: readonly User[]): string[] {
  return (
    users.filter(user => user.active !== false) // Assuming undefined or false is considered not active
      .map(user => user.email.toLowerCase())
  );
}
```