```ts
/** The lowercased addresses of the active users, in order. */
export function activeEmails(users: readonly User[]): string[] {
  /**
   * Returns the lowercased addresses of the active users, in input order.
   *
   * @param {readonly User[]} users - An array of User objects.
   * @returns {string[]} The lowercased addresses of the active users, in order.
   */
  return users.filter(user => user.active).map(user => user.email.toLowerCase());
}
```