# app/mailers/user_mailer.rb
class UserMailer < ActionMailer::Base
  default from: "some-email@your-domain.ext"
  def password_changed(id)
    @user = User.find(id)
    mail to: @user.email, subject: "Your password has changed"
  end
end

# app/models/user.rb
class User < ActiveRecord::Base
  after_update :send_password_change_email, if: :needs_password_change_email?
  private
  # Change the logic here depending on how you use Devise.
  # For example, if you allow users to be created with just an email,
  # then this will always return true, so you'll need another thing to
  # check instead of `persisted?`
  #
  # The idea is that you want to differentiate between users who are signing
  # up for the first time (because `encrypted_password_changed?` will be true
  # for them), and those who are changing their password after having created
  # it for the first time.
  def needs_password_change_email?
    encrypted_password_changed? && persisted?
  end
   
  def send_password_change_email
    UserMailer.password_changed(id).deliver
  end
end