HerokuのSendgridアドオンを用いてRailsでユーザ作成したい

前提・実現したいこと

HerokuのSendgridアドオンを用いてRailsでユーザ作成時のメール認証機能を実装したいと考えています。

＊当方RailsやHerokuを用いた開発経験が無く、手探り状態である事をご了承ください。

###やった事
postmanを用いて下記のJSON形式のユーザ情報を投げてテストを行いましたがエラーが出ました

JSON
1{
2	"user":{
3		"user_name":"****",
4		"email":"****@icloud.com",
5		"password":"rhcp21",
6		"password_confirmation":"rhcp21"
7		
8	}
9}

発生している問題・エラーメッセージ

500:Internal Server Error
Net::SMTPFatalError: 550 Unauthenticated senders not allowed
エラーの詳細は以下に示します
匿名性を保持する為、*********@icloud.comと表記させて頂きます。

Started POST "/users" for ::1 at 2019-11-09 11:43:42 +0900
Processing by UsersController#create as */*
  Parameters: {"user"=>{"user_name"=>"****", "email"=>"****@icloud.com", "password"=>"[FILTERED]", "password_confirmation"=>"[FILTERED]"}}
   (1.7ms)  BEGIN
  ↳ app/controllers/users_controller.rb:26
  User Exists (2.5ms)  SELECT  1 AS one FROM "users" WHERE LOWER("users"."email") = LOWER($1) LIMIT $2  [["email", "****@icloud.com"], ["LIMIT", 1]]
  ↳ app/controllers/users_controller.rb:26
  User Create (2.3ms)  INSERT INTO "users" ("email", "password_digest", "user_name", "created_at", "updated_at") VALUES ($1, $2, $3, $4, $5) RETURNING "id"  [["email", "s
houkitsuda@icloud.com"], ["password_digest", "$2a$12$muT2pAzq4SuIOzLea77.euur1wVwgoiV8JtgXmXDp7sESiPy8W1va"], ["user_name", "rhcp21"], ["created_at", "2019-11-09 11:43:42
.662938"], ["updated_at", "2019-11-09 11:43:42.662938"]]
  ↳ app/controllers/users_controller.rb:26
   (7.9ms)  COMMIT
  ↳ app/controllers/users_controller.rb:26

  Rendering user_mailer/account_activation.html.erb within layouts/mailer
  Rendered user_mailer/account_activation.html.erb within layouts/mailer (7.9ms)
UserMailer#account_activation: processed outbound mail in 15.4ms
Completed 500 Internal Server Error in 328ms (ActiveRecord: 14.4ms)
ActionView::Template::Error (undefined method `name' for #<User:0x00007fa790aef210>):
    1: <h1>Sample App</h1>
    2:
    3: <p>Hi <%= @user.name %>,</p>
    4:
    5: <p>
    6: Welcome to the Sample App! Click on the link below to activate your account:


app/views/user_mailer/account_activation.html.erb:3:in `_app_views_user_mailer_account_activation_html_erb___3715546868340462326_70178831968000'
app/mailers/user_mailer.rb:5:in `account_activation'
app/models/user.rb:56:in `send_activation_email'
app/controllers/users_controller.rb:27:in `create'

該当のソースコード

config/development.rb

ruby
1  config.action_mailer.raise_delivery_errors = true
2  config.action_mailer.delivery_method = :smtp
3  host ='https://aqueous-escarpment-97262.herokuapp.com'
4  config.action_mailer.default_url_options = { host: host }
5  ActionMailer::Base.smtp_settings = {
6    :address        => 'smtp.sendgrid.net',
7    :port           => '587',
8    :authentication => :plain,
9    :user_name      => ENV['SENDGRID_USERNAME'],
10    :password       => ENV['SENDGRID_PASSWORD'],
11    :domain         => 'heroku.com',
12    :enable_starttls_auto => true

account_activation.html.erb

erb
1<h1>Sample App</h1>
2
3<p>Hi <%= @user.name %>,</p>
4
5<p>
6Welcome to the Sample App! Click on the link below to activate your account:
7</p>
8
9<%= link_to "Activate", edit_account_activation_url(@user.activation_token,
10                                                    email: @user.email) %>
11

user_mailer.rb

ruby
1class UserMailer < ApplicationMailer
2
3  def account_activation(user)#5行
4    @user = user
5    mail to: user.email, subject: "Account activation"
6  end
7
8  # def password_reset(user)
9  #   @user = user
10  #   mail to: user.email, subject: "Password reset"
11  # end
12end
13

user.rb

ruby
1   # Sends activation email.
2    def send_activation_email
3      UserMailer.account_activation(self).deliver_now#56行
4    end

users_controller.rb

ruby
1  def create
2    @user = User.new(user_params)
3    # @users = params[:user].permit(:email, :password, :password_confirmation)
4    # User.new(@users)
5
6    if @user.save
7     @user.send_activation_email#27行
8     render json: { status:  "Please check your email to activate your account." }
9      # flash[:info] = "Please check your email to activate your account."
10      # redirect_to root_url
11      render json: { status: 'success', data: @user }
12    else
13      render json: { status: 'error!', data: @user.errors }
14    end
15  end