Userのメイラーを追加します。
このメイラーはUsersコントローラのcreateアクションで有効化リンクをメール送信するために使います。

ubuntu:~/environment/my_app (account-activation) $ rails generate mailer UserMailer account_activation password_reset
Running via Spring preloader in process 12921
      create  app/mailers/user_mailer.rb
      invoke  erb
      create    app/views/user_mailer
      create    app/views/user_mailer/account_activation.text.erb
      create    app/views/user_mailer/account_activation.html.erb
      create    app/views/user_mailer/password_reset.text.erb
      create    app/views/user_mailer/password_reset.html.erb
      invoke  test_unit
      create    test/mailers/user_mailer_test.rb
      create    test/mailers/previews/user_mailer_preview.rb

###app/views/user_mailer/account_activation.text.erb

app/views/user_mailer/account_activation.text.erb

###app/views/user_mailer/account_activation.html.erb

app/views/user_mailer/account_activation.html.erb

<h1>Sample App</h1>

<p>Hi <%= @user.name %>,</p>

<p>
Welcome to the Sample App! Click on the link below to activate your account:
</p>

<%= link_to "Activate", edit_account_activation_url(@user.activation_token,
                                                    email: @user.email) %>

###app/mailers/application_mailer.rb

app/mailers/application_mailer.rb

class ApplicationMailer < ActionMailer::Base
  default from: 'from@example.com'
  layout 'mailer'
end

###app/mailers/user_mailer.rb

app/mailers/user_mailer.rb

class UserMailer < ApplicationMailer
  
  def account_activation(user)
    @user = user
    # インスタンス引数
    mail to: user.email, subject: "Account activation"
    # 
  end

  def account_activation
    @greeting = "こんにちは！"

    mail to: "to@example.org"
  end

  # Subject can be set in your I18n file at config/locales/en.yml
  # with the following lookup:
  #
  #   en.user_mailer.password_reset.subject
  #
  def password_reset
    @greeting = "こんにちは！"

    mail to: "to@example.org"
  end
end

###config/environments/development.rb

config/environments/development.rb

Rails.application.configure do
  # Settings specified here will take precedence over those in config/application.rb.

  # In the development environment your application's code is reloaded on
  # every request. This slows down response time but is perfect for development
  # since you don't have to restart the web server when you make code changes.
  config.cache_classes = false

  # Do not eager load code on boot.
  config.eager_load = false

  # Show full error reports.
  config.consider_all_requests_local = true

  # Enable/disable caching. By default caching is disabled.
  # Run rails dev:cache to toggle caching.
  if Rails.root.join('tmp', 'caching-dev.txt').exist?
    config.action_controller.perform_caching = true
    config.action_controller.enable_fragment_cache_logging = true

    config.cache_store = :memory_store
    config.public_file_server.headers = {
      'Cache-Control' => "public, max-age=#{2.days.to_i}"
    }
  else
    config.action_controller.perform_caching = false

    config.cache_store = :null_store
  end

  # Store uploaded files on the local file system (see config/storage.yml for options).
  config.active_storage.service = :local

  # Don't care if the mailer can't send.
  config.action_mailer.raise_delivery_errors = false
  
  host = 'example.com' # ここをコピペすると失敗します。自分の環境のホストに変えてください。
  # クラウドIDEの場合は以下をお使いください
  config.action_mailer.default_url_options = { host: host, protocol: 'https' }
  # localhostで開発している場合は以下をお使いください
  # config.action_mailer.default_url_options = { host: host, protocol: 'http' }
  config.action_mailer.perform_caching = false

  # Print deprecation notices to the Rails logger.
  config.active_support.deprecation = :log

  # Raise an error on page load if there are pending migrations.
  config.active_record.migration_error = :page_load

  # Highlight code that triggered database queries in logs.
  config.active_record.verbose_query_logs = true

  # Debug mode disables concatenation and preprocessing of assets.
  # This option may cause significant delays in view rendering with a large
  # number of complex assets.
  config.assets.debug = true

  # Suppress logger output for asset requests.
  config.assets.quiet = true

  # Raises error for missing translations.
  # config.action_view.raise_on_missing_translations = true

  # Use an evented file watcher to asynchronously detect changes in source code,
  # routes, locales, etc. This feature depends on the listen gem.
  config.file_watcher = ActiveSupport::EventedFileUpdateChecker
  
  # Cloud9 への接続を許可する
  config.hosts.clear
end

###test/mailers/previews/user_mailer_preview.rb

test/mailers/previews/user_mailer_preview.rb

# Preview all emails at http://localhost:3000/rails/mailers/user_mailer
class UserMailerPreview < ActionMailer::Preview

  # Preview this email at http://localhost:3000/rails/mailers/user_mailer/account_activation
  def account_activation
  # アカウント作成をされたら自分にメールが送信されるためのメソッド
    user = User.first
   　　# db最初のユーザーを代入
    user.activation_token = User.new_token
    # 最初のユーザーに新しいトークンを生成
    UserMailer.account_activation(user)
    # 有効化のためにメールを送信する
  end

  # Preview this email at http://localhost:3000/rails/mailers/user_mailer/password_reset
  def password_reset
    UserMailer.password_reset
  end
end

###test/mailers/user_mailer_test.rb

test/mailers/user_mailer_test.rb

require 'test_helper'

class UserMailerTest < ActionMailer::TestCase

  test "account_activation" do
  # 登録時送信メールのテスト
    user = users(:michael)
    # テストユーザー
    user.activation_token = User.new_token
    # 新しいトークンを作成　有効化トークンに代入
    mail = UserMailer.account_activation(user)
    # 登録用のメールを送信する
    assert_equal "Account activation", mail.subject
    # 内容があっているか確認
    assert_equal [user.email], mail.to
    assert_equal ["noreply@example.com"], mail.from
    assert_match user.name,               mail.body.encoded
    assert_match user.activation_token,   mail.body.encoded
    assert_match CGI.escape(user.email),  mail.body.encoded
  end
end

###config/environments/test.rb

config/environments/test.rb

# The test environment is used exclusively to run your application's
# test suite. You never need to work with it otherwise. Remember that
# your test database is "scratch space" for the test suite and is wiped
# and recreated between test runs. Don't rely on the data there!

Rails.application.configure do
  # Settings specified here will take precedence over those in config/application.rb.

  config.cache_classes = false
  config.action_view.cache_template_loading = true

  # Do not eager load code on boot. This avoids loading your whole application
  # just for the purpose of running a single test. If you are using a tool that
  # preloads Rails for running tests, you may have to set it to true.
  config.eager_load = false

  # Configure public file server for tests with Cache-Control for performance.
  config.public_file_server.enabled = true
  config.public_file_server.headers = {
    'Cache-Control' => "public, max-age=#{1.hour.to_i}"
  }

  # Show full error reports and disable caching.
  config.consider_all_requests_local       = true
  config.action_controller.perform_caching = false
  config.cache_store = :null_store

  # Raise exceptions instead of rendering exception templates.
  config.action_dispatch.show_exceptions = false

  # Disable request forgery protection in test environment.
  config.action_controller.allow_forgery_protection = false

  # Store uploaded files on the local file system in a temporary directory.
  config.active_storage.service = :test

  config.action_mailer.perform_caching = false

  # Tell Action Mailer not to deliver emails to the real world.
  # The :test delivery method accumulates sent emails in the
  # ActionMailer::Base.deliveries array.
  config.action_mailer.delivery_method = :test
  config.action_mailer.default_url_options = { host: 'example.com' }

  # Print deprecation notices to the stderr.
  config.active_support.deprecation = :stderr

  # Raises error for missing translations.
  # config.action_view.raise_on_missing_translations = true
end

###app/controllers/users_controller.rb

app/controllers/users_controller.rb

class UsersController < ApplicationController
  before_action :logged_in_user, only: [:index, :edit, :update, :destroy]
  before_action :correct_user,   only: [:edit, :update]
  before_action :admin_user,     only: :destroy
  
  def index
    @users = User.paginate(page: params[:page])
  end

  def show
    @user = User.find(params[:id])
  end
 
  def new
    @user = User.new
  end
  
  def create
    @user = User.new(user_params) 
    if @user.save
      UserMailer.account_activation(@user).deliver_now
      # deliver_now
        # メール自身が送信対象であることをdeliver_nowやdeliver_laterに伝えます
      flash[:info] = "メールをチェックしてアカウントを有効にしてください。"
      redirect_to root_url
      # 自動的に移動する
    else
      render 'new'
    end
  end
  
  def edit
    @user = User.find(params[:id])
  end
  
  def update
    @user = User.find(params[:id])
    if @user.update(user_params)
      flash[:success] = "Profile updated"
      redirect_to @user
    else
      render 'edit'
    end
  end

  def destroy
    User.find(params[:id]).destroy
    flash[:success] = "User deleted"
    redirect_to users_url
  end
  
  private
  
    def user_params
      params.require(:user).permit(:name, :email, :password, 
                                     :password_confirmation)
    end
    
    def logged_in_user
      unless logged_in?
        store_location
        flash[:danger] = "Please log in."
        redirect_to login_url
      end
    end
    
    # 正しいユーザーかどうか確認
    def correct_user
      @user = User.find(params[:id])
      redirect_to(root_url) unless @user == current_user
    end
    
    # 管理者かどうか確認
    def admin_user
      redirect_to(root_url) unless current_user.admin?
    end
end