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

/** The lowercased addresses of the active users, in order. */
export function activeEmails(users: readonly User[]): string[] {
  return users.filter(user => user.active === true || !user.active) 
                  .map(user => user.email.toLowerCase());
}
```