Action Mailer 基础

This guide provides you with all you need to get started in sending emails from your application, and many internals of Action Mailer. It also covers how to test your mailers.

After reading this guide, you will know:

How to send email within a Rails application.
How to generate and edit an Action Mailer class and mailer view.
How to configure Action Mailer for your environment.
How to test your Action Mailer classes.

What is Action Mailer?

Action Mailer allows you to send emails from your application using mailer classes and views.

Mailers are Similar to Controllers

They inherit from ActionMailer::Base and live in app/mailers. Mailers also work very similarly to controllers. Some examples of similarities are enumerated below. Mailers have:

Actions, and also, associated views that appear in app/views.
Instance variables that are accessible in views.
The ability to utilise layouts and partials.
The ability to access a params hash.

Sending Emails

This section will provide a step-by-step guide to creating a mailer and its views.

Walkthrough to Generating a Mailer

Create the Mailer

$ bin/rails generate mailer User
create  app/mailers/user_mailer.rb
create  app/mailers/application_mailer.rb
invoke  erb
create    app/views/user_mailer
create    app/views/layouts/mailer.text.erb
create    app/views/layouts/mailer.html.erb
invoke  test_unit
create    test/mailers/user_mailer_test.rb
create    test/mailers/previews/user_mailer_preview.rb

# app/mailers/application_mailer.rb
class ApplicationMailer < ActionMailer::Base
  default from: "from@example.com"
  layout 'mailer'
end

# app/mailers/user_mailer.rb
class UserMailer < ApplicationMailer
end

As you can see, you can generate mailers just like you use other generators with Rails.

If you didn’t want to use a generator, you could create your own file inside of app/mailers, just make sure that it inherits from ActionMailer::Base:

class MyMailer < ActionMailer::Base
end

Edit the Mailer

Mailers have methods called “actions” and they use views to structure their content. Where a controller generates content like HTML to send back to the client, a Mailer creates a message to be delivered via email.

app/mailers/user_mailer.rb contains an empty mailer:

class UserMailer < ApplicationMailer
end

Let’s add a method called welcome_email, that will send an email to the user’s registered email address:

class UserMailer < ApplicationMailer
  default from: 'notifications@example.com'

  def welcome_email
    @user = params[:user]
    @url  = 'http://example.com/login'
    mail(to: @user.email, subject: 'Welcome to My Awesome Site')
  end
end

Here is a quick explanation of the items presented in the preceding method. For a full list of all available options, please have a look further down at the Complete List of Action Mailer user-settable attributes section.

The default method sets default values for all emails sent from this mailer. In this case, we use it to set the :from header value for all messages in this class. This can be overridden on a per-email basis.
The mail method creates the actual email message. We use it to specify the values of headers like :to and :subject per email.

Create a Mailer View

Create a file called welcome_email.html.erb in app/views/user_mailer/. This will be the template used for the email, formatted in HTML:

<!DOCTYPE html>
<html>
  <head>
    <meta content='text/html; charset=UTF-8' http-equiv='Content-Type' />
  </head>
  <body>
    <h1>Welcome to example.com, <%= @user.name %></h1>
    <p>
      You have successfully signed up to example.com,
      your username is: <%= @user.login %>.<br>
    </p>
    <p>
      To login to the site, just follow this link: <%= @url %>.
    </p>
    <p>Thanks for joining and have a great day!</p>
  </body>
</html>

Let’s also make a text part for this email. Not all clients prefer HTML emails, and so sending both is best practice. To do this, create a file called welcome_email.text.erb in app/views/user_mailer/:

Welcome to example.com, <%= @user.name %>
===============================================

You have successfully signed up to example.com,
your username is: <%= @user.login %>.

To login to the site, just follow this link: <%= @url %>.

Thanks for joining and have a great day!

When you call the mail method now, Action Mailer will detect the two templates (text and HTML) and automatically generate a multipart/alternative email.

Calling the Mailer

Mailers are really just another way to render a view. Instead of rendering a view and sending it over the HTTP protocol, they are sending it out through the email protocols instead. Due to this, it makes sense to have your controller tell the Mailer to send an email when a user is successfully created.

Setting this up is simple.

First, let’s create a User scaffold:

$ bin/rails generate scaffold user name email login
$ bin/rails db:migrate

Now that we have a user model to play with, we will edit the app/controllers/users_controller.rb file, make it instruct the UserMailer to deliver an email to the newly created user by editing the create action and inserting a call to UserMailer.with(user: @user).welcome_email right after the user is successfully saved.

We will enqueue the email to be sent by using deliver_later, which is backed by Active Job. That way, the controller action can continue without waiting for the send to complete.

class UsersController < ApplicationController
  # ...

  # POST /users or /users.json
  def create
    @user = User.new(user_params)

    respond_to do |format|
      if @user.save
        # Tell the UserMailer to send a welcome email after save
        UserMailer.with(user: @user).welcome_email.deliver_later

        format.html { redirect_to(@user, notice: 'User was successfully created.') }
        format.json { render json: @user, status: :created, location: @user }
      else
        format.html { render action: 'new' }
        format.json { render json: @user.errors, status: :unprocessable_entity }
      end
    end
  end

  # ...
end

NOTE: Active Job’s default behavior is to execute jobs via the :async adapter. So, you can use deliver_later to send emails asynchronously. Active Job’s default adapter runs jobs with an in-process thread pool. It’s well-suited for the development/test environments, since it doesn’t require any external infrastructure, but it’s a poor fit for production since it drops pending jobs on restart. If you need a persistent backend, you will need to use an Active Job adapter that has a persistent backend (Sidekiq, Resque, etc).

If you want to send emails right away (from a cronjob for example) just call deliver_now:

class SendWeeklySummary
  def run
    User.find_each do |user|
      UserMailer.with(user: user).weekly_summary.deliver_now
    end
  end
end

Any key-value pair passed to with just becomes the params for the mailer action. So with(user: @user, account: @user.account) makes params[:user] and params[:account] available in the mailer action. Just like controllers have params.

The method welcome_email returns an ActionMailer::MessageDelivery object which can then be told to deliver_now or deliver_later to send itself out. The ActionMailer::MessageDelivery object is a wrapper around a Mail::Message. If you want to inspect, alter, or do anything else with the Mail::Message object you can access it with the message method on the ActionMailer::MessageDelivery object.

Auto Encoding Header Values

Action Mailer handles the auto encoding of multibyte characters inside of headers and bodies.

For more complex examples such as defining alternate character sets or self-encoding text first, please refer to the Mail library.

Complete List of Action Mailer Methods

There are just three methods that you need to send pretty much any email message:

headers - Specifies any header on the email you want. You can pass a hash of header field names and value pairs, or you can call headers[:field_name] = 'value'.
attachments - Allows you to add attachments to your email. For example, attachments['file-name.jpg'] = File.read('file-name.jpg').
mail - Creates the actual email itself. You can pass in headers as a hash to the mail method as a parameter. mail will create an email — either plain text or multipart — depending on what email templates you have defined.

Adding Attachments

Action Mailer makes it very easy to add attachments.

Pass the file name and content and Action Mailer and the Mail gem will automatically guess the mime_type, set the encoding, and create the attachment.
```
  attachments['filename.jpg'] = File.read('/path/to/filename.jpg')
```
When the mail method will be triggered, it will send a multipart email with an attachment, properly nested with the top level being multipart/mixed and the first part being a multipart/alternative containing the plain text and HTML email messages.

NOTE: Mail will automatically Base64 encode an attachment. If you want something different, encode your content and pass in the encoded content and encoding in a Hash to the attachments method.

Pass the file name and specify headers and content and Action Mailer and Mail will use the settings you pass in.

  encoded_content = SpecialEncode(File.read('/path/to/filename.jpg'))
  attachments['filename.jpg'] = {
    mime_type: 'application/gzip',
    encoding: 'SpecialEncoding',
    content: encoded_content
  }

NOTE: If you specify an encoding, Mail will assume that your content is already encoded and not try to Base64 encode it.

Making Inline Attachments

Action Mailer 3.0 makes inline attachments, which involved a lot of hacking in pre 3.0 versions, much simpler and trivial as they should be.

First, to tell Mail to turn an attachment into an inline attachment, you just call #inline on the attachments method within your Mailer:
```
  def welcome
    attachments.inline['image.jpg'] = File.read('/path/to/image.jpg')
  end
```
Then in your view, you can just reference attachments as a hash and specify which attachment you want to show, calling url on it and then passing the result into the image_tag method:
```
  <p>Hello there, this is our image</p>

  <%= image_tag attachments['image.jpg'].url %>
```
As this is a standard call to image_tag you can pass in an options hash after the attachment URL as you could for any other image:
```
  <p>Hello there, this is our image</p>

  <%= image_tag attachments['image.jpg'].url, alt: 'My Photo', class: 'photos' %>
```

Sending Email to Multiple Recipients

It is possible to send email to one or more recipients in one email (e.g., informing all admins of a new signup) by setting the list of emails to the :to key. The list of emails can be an array of email addresses or a single string with the addresses separated by commas.

class AdminMailer < ApplicationMailer
  default to: -> { Admin.pluck(:email) },
          from: 'notification@example.com'

  def new_registration(user)
    @user = user
    mail(subject: "New User Signup: #{@user.email}")
  end
end

The same format can be used to set carbon copy (Cc:) and blind carbon copy (Bcc:) recipients, by using the :cc and :bcc keys respectively.

Sending Email with Name

Sometimes you wish to show the name of the person instead of just their email address when they receive the email. You can use email_address_with_name for that:

def welcome_email
  @user = params[:user]
  mail(
    to: email_address_with_name(@user.email, @user.name),
    subject: 'Welcome to My Awesome Site'
  )
end

The same technique works to specify a sender name:

class UserMailer < ApplicationMailer
  default from: email_address_with_name('notification@example.com', 'Example Company Notifications')
end

If the name is a blank string, it returns just the address.

Mailer Views

Mailer views are located in the app/views/name_of_mailer_class directory. The specific mailer view is known to the class because its name is the same as the mailer method. In our example from above, our mailer view for the welcome_email method will be in app/views/user_mailer/welcome_email.html.erb for the HTML version and welcome_email.text.erb for the plain text version.

To change the default mailer view for your action you do something like:

class UserMailer < ApplicationMailer
  default from: 'notifications@example.com'

  def welcome_email
    @user = params[:user]
    @url  = 'http://example.com/login'
    mail(to: @user.email,
         subject: 'Welcome to My Awesome Site',
         template_path: 'notifications',
         template_name: 'another')
  end
end

In this case, it will look for templates at app/views/notifications with name another. You can also specify an array of paths for template_path, and they will be searched in order.

If you want more flexibility you can also pass a block and render specific templates or even render inline or text without using a template file:

class UserMailer < ApplicationMailer
  default from: 'notifications@example.com'

  def welcome_email
    @user = params[:user]
    @url  = 'http://example.com/login'
    mail(to: @user.email,
         subject: 'Welcome to My Awesome Site') do |format|
      format.html { render 'another_template' }
      format.text { render plain: 'Render text' }
    end
  end
end

This will render the template ‘another_template.html.erb’ for the HTML part and use the rendered text for the text part. The render command is the same one used inside of Action Controller, so you can use all the same options, such as :text, :inline, etc.

If you would like to render a template located outside of the default app/views/mailer_name/ directory, you can apply the prepend_view_path, like so:

class UserMailer < ApplicationMailer
  prepend_view_path "custom/path/to/mailer/view"

  # This will try to load "custom/path/to/mailer/view/welcome_email" template
  def welcome_email
    # ...
  end
end

You can also consider using the append_view_path method.

Caching Mailer View

You can perform fragment caching in mailer views like in application views using the cache method.

<% cache do %>
  <%= @company.name %>
<% end %>

And to use this feature, you need to configure your application with this:

config.action_mailer.perform_caching = true

Fragment caching is also supported in multipart emails. Read more about caching in the Rails caching guide.

Action Mailer Layouts

Just like controller views, you can also have mailer layouts. The layout name needs to be the same as your mailer, such as user_mailer.html.erb and user_mailer.text.erb to be automatically recognized by your mailer as a layout.

To use a different file, call layout in your mailer:

class UserMailer < ApplicationMailer
  layout 'awesome' # use awesome.(html|text).erb as the layout
end

Just like with controller views, use yield to render the view inside the layout.

You can also pass in a layout: 'layout_name' option to the render call inside the format block to specify different layouts for different formats:

class UserMailer < ApplicationMailer
  def welcome_email
    mail(to: params[:user].email) do |format|
      format.html { render layout: 'my_layout' }
      format.text
    end
  end
end

Will render the HTML part using the my_layout.html.erb file and the text part with the usual user_mailer.text.erb file if it exists.

Previewing Emails

Action Mailer previews provide a way to see how emails look by visiting a special URL that renders them. In the above example, the preview class for UserMailer should be named UserMailerPreview and located in test/mailers/previews/user_mailer_preview.rb. To see the preview of welcome_email, implement a method that has the same name and call UserMailer.welcome_email:

class UserMailerPreview < ActionMailer::Preview
  def welcome_email
    UserMailer.with(user: User.first).welcome_email
  end
end

Then the preview will be available in http://localhost:3000/rails/mailers/user_mailer/welcome_email.

If you change something in app/views/user_mailer/welcome_email.html.erb or the mailer itself, it’ll automatically reload and render it so you can visually see the new style instantly. A list of previews are also available in http://localhost:3000/rails/mailers.

By default, these preview classes live in test/mailers/previews. This can be configured using the preview_paths option. For example, if you want to add lib/mailer_previews to it, you can configure it in config/application.rb:

config.action_mailer.preview_paths << "#{Rails.root}/lib/mailer_previews"

Generating URLs in Action Mailer Views

Unlike controllers, the mailer instance doesn’t have any context about the incoming request so you’ll need to provide the :host parameter yourself.

As the :host usually is consistent across the application you can configure it globally in config/application.rb:

config.action_mailer.default_url_options = { host: 'example.com' }

Because of this behavior, you cannot use any of the *_path helpers inside of an email. Instead, you will need to use the associated *_url helper. For example instead of using

<%= link_to 'welcome', welcome_path %>

You will need to use:

<%= link_to 'welcome', welcome_url %>

By using the full URL, your links will now work in your emails.

Generating URLs with `url_for`

url_for generates a full URL by default in templates.

If you did not configure the :host option globally make sure to pass it to url_for.

<%= url_for(host: 'example.com',
            controller: 'welcome',
            action: 'greeting') %>

Generating URLs with Named Routes

Email clients have no web context and so paths have no base URL to form complete web addresses. Thus, you should always use the *_url variant of named route helpers.

If you did not configure the :host option globally make sure to pass it to the URL helper.

<%= user_url(@user, host: 'example.com') %>

NOTE: non-GET links require rails-ujs or jQuery UJS, and won’t work in mailer templates. They will result in normal GET requests.

Adding Images in Action Mailer Views

Unlike controllers, the mailer instance doesn’t have any context about the incoming request so you’ll need to provide the :asset_host parameter yourself.

As the :asset_host usually is consistent across the application you can configure it globally in config/application.rb:

config.asset_host = 'http://example.com'

Now you can display an image inside your email.

<%= image_tag 'image.jpg' %>

Sending Multipart Emails

Action Mailer will automatically send multipart emails if you have different templates for the same action. So, for our UserMailer example, if you have welcome_email.text.erb and welcome_email.html.erb in app/views/user_mailer, Action Mailer will automatically send a multipart email with the HTML and text versions setup as different parts.

The order of the parts getting inserted is determined by the :parts_order inside of the ActionMailer::Base.default method.

Sending Emails with Dynamic Delivery Options

If you wish to override the default delivery options (e.g. SMTP credentials) while delivering emails, you can do this using delivery_method_options in the mailer action.

class UserMailer < ApplicationMailer
  def welcome_email
    @user = params[:user]
    @url  = user_url(@user)
    delivery_options = { user_name: params[:company].smtp_user,
                         password: params[:company].smtp_password,
                         address: params[:company].smtp_host }
    mail(to: @user.email,
         subject: "Please see the Terms and Conditions attached",
         delivery_method_options: delivery_options)
  end
end

Sending Emails without Template Rendering

There may be cases in which you want to skip the template rendering step and supply the email body as a string. You can achieve this using the :body option. In such cases don’t forget to add the :content_type option. Rails will default to text/plain otherwise.

class UserMailer < ApplicationMailer
  def welcome_email
    mail(to: params[:user].email,
         body: params[:email_body],
         content_type: "text/html",
         subject: "Already rendered!")
  end
end

Action Mailer Callbacks

Action Mailer allows for you to specify a before_action, after_action and around_action.

Filters can be specified with a block or a symbol to a method in the mailer class similar to controllers.
You could use a before_action to set instance variables, populate the mail object with defaults, or insert default headers and attachments.

class InvitationsMailer < ApplicationMailer
  before_action :set_inviter_and_invitee
  before_action { @account = params[:inviter].account }

  default to:       -> { @invitee.email_address },
          from:     -> { common_address(@inviter) },
          reply_to: -> { @inviter.email_address_with_name }

  def account_invitation
    mail subject: "#{@inviter.name} invited you to their Basecamp (#{@account.name})"
  end

  def project_invitation
    @project    = params[:project]
    @summarizer = ProjectInvitationSummarizer.new(@project.bucket)

    mail subject: "#{@inviter.name.familiar} added you to a project in Basecamp (#{@account.name})"
  end

  private

  def set_inviter_and_invitee
    @inviter = params[:inviter]
    @invitee = params[:invitee]
  end
end

You could use an after_action to do similar setup as a before_action but using instance variables set in your mailer action.
Using an after_action callback also enables you to override delivery method settings by updating mail.delivery_method.settings.

class UserMailer < ApplicationMailer
  before_action { @business, @user = params[:business], params[:user] }

  after_action :set_delivery_options,
               :prevent_delivery_to_guests,
               :set_business_headers

  def feedback_message
  end

  def campaign_message
  end

  private

    def set_delivery_options
      # You have access to the mail instance,
      # @business and @user instance variables here
      if @business && @business.has_smtp_settings?
        mail.delivery_method.settings.merge!(@business.smtp_settings)
      end
    end

    def prevent_delivery_to_guests
      if @user && @user.guest?
        mail.perform_deliveries = false
      end
    end

    def set_business_headers
      if @business
        headers["X-SMTPAPI-CATEGORY"] = @business.code
      end
    end
end

Mailer Filters abort further processing if body is set to a non-nil value.

Using Action Mailer Helpers

Action Mailer inherits from AbstractController, so you have access to most of the same helpers as you do in Action Controller.

There are also some Action Mailer-specific helper methods available in ActionMailer::MailHelper. For example, these allow accessing the mailer instance from your view with mailer, and accessing the message as message:

<%= stylesheet_link_tag mailer.name.underscore %>
<h1><%= message.subject %></h1>

Action Mailer Configuration

The following configuration options are best made in one of the environment files (environment.rb, production.rb, etc…)

Configuration	Description
`logger`	Generates information on the mailing run if available. Can be set to `nil` for no logging. Compatible with both Ruby’s own `Logger` and `Log4r` loggers.
`smtp_settings`	Allows detailed configuration for `:smtp` delivery method:<ul><li>`:address` - Allows you to use a remote mail server. Just change it from its default `"localhost"` setting.</li><li>`:port` - On the off chance that your mail server doesn’t run on port 25, you can change it.</li><li>`:domain` - If you need to specify a HELO domain, you can do it here.</li><li>`:user_name` - If your mail server requires authentication, set the username in this setting.</li><li>`:password` - If your mail server requires authentication, set the password in this setting.</li><li>`:authentication` - If your mail server requires authentication, you need to specify the authentication type here. This is a symbol and one of `:plain` (will send the password in the clear), `:login` (will send password Base64 encoded) or `:cram_md5` (combines a Challenge/Response mechanism to exchange information and a cryptographic Message Digest 5 algorithm to hash important information)</li><li>`:enable_starttls` - Use STARTTLS when connecting to your SMTP server and fail if unsupported. Defaults to `false`.</li><li>`:enable_starttls_auto` - Detects if STARTTLS is enabled in your SMTP server and starts to use it. Defaults to `true`.</li><li>`:openssl_verify_mode` - When using TLS, you can set how OpenSSL checks the certificate. This is really useful if you need to validate a self-signed and/or a wildcard certificate. You can use the name of an OpenSSL verify constant (‘none’ or ‘peer’) or directly the constant (`OpenSSL::SSL::VERIFY_NONE` or `OpenSSL::SSL::VERIFY_PEER`).</li><li>`:ssl/:tls` - Enables the SMTP connection to use SMTP/TLS (SMTPS: SMTP over direct TLS connection)</li><li>`:open_timeout` - Number of seconds to wait while attempting to open a connection.</li><li>`:read_timeout` - Number of seconds to wait until timing-out a read(2) call.</li></ul>
`sendmail_settings`	Allows you to override options for the `:sendmail` delivery method.<ul><li>`:location` - The location of the sendmail executable. Defaults to `/usr/sbin/sendmail`.</li><li>`:arguments` - The command line arguments to be passed to sendmail. Defaults to `["-i"]`.</li></ul>
`raise_delivery_errors`	Whether or not errors should be raised if the email fails to be delivered. This only works if the external email server is configured for immediate delivery. Defaults to `true`.
`delivery_method`	Defines a delivery method. Possible values are:<ul><li>`:smtp` (default), can be configured by using `config.action_mailer.smtp_settings`.</li><li>`:sendmail`, can be configured by using `config.action_mailer.sendmail_settings`.</li><li>`:file`: save emails to files; can be configured by using `config.action_mailer.file_settings`.</li><li>`:test`: save emails to `ActionMailer::Base.deliveries` array.</li></ul>See API docs for more info.
`perform_deliveries`	Determines whether deliveries are actually carried out when the `deliver` method is invoked on the Mail message. By default they are, but this can be turned off to help functional testing. If this value is `false`, `deliveries` array will not be populated even if `delivery_method` is `:test`.
`deliveries`	Keeps an array of all the emails sent out through the Action Mailer with delivery_method :test. Most useful for unit and functional testing.
`delivery_job`	The job class used with `deliver_later`. Defaults to `ActionMailer::MailDeliveryJob`.
`deliver_later_queue_name`	The name of the queue used with `deliver_later`.
`default_options`	Allows you to set default values for the `mail` method options (`:from`, `:reply_to`, etc.).

For a complete writeup of possible configurations see the Configuring Action Mailer in our Configuring Rails Applications guide.

Example Action Mailer Configuration

An example would be adding the following to your appropriate config/environments/$RAILS_ENV.rb file:

config.action_mailer.delivery_method = :sendmail
# Defaults to:
# config.action_mailer.sendmail_settings = {
#   location: '/usr/sbin/sendmail',
#   arguments: %w[ -i ]
# }
config.action_mailer.perform_deliveries = true
config.action_mailer.raise_delivery_errors = true
config.action_mailer.default_options = {from: 'no-reply@example.com'}

Action Mailer Configuration for Gmail

Action Mailer uses the Mail gem and accepts similar configuration. Add this to your config/environments/$RAILS_ENV.rb file to send via Gmail:

config.action_mailer.delivery_method = :smtp
config.action_mailer.smtp_settings = {
  address:         'smtp.gmail.com',
  port:            587,
  domain:          'example.com',
  user_name:       '<username>',
  password:        '<password>',
  authentication:  'plain',
  enable_starttls: true,
  open_timeout:    5,
  read_timeout:    5 }

If you are using an old version of the Mail gem (2.6.x or earlier), use enable_starttls_auto instead of enable_starttls.

NOTE: Google blocks sign-ins from apps it deems less secure. You can change your Gmail settings here to allow the attempts. If your Gmail account has 2-factor authentication enabled, then you will need to set an app password and use that instead of your regular password.

Mailer Testing

You can find detailed instructions on how to test your mailers in the testing guide.

Intercepting and Observing Emails

Action Mailer provides hooks into the Mail observer and interceptor methods. These allow you to register classes that are called during the mail delivery life cycle of every email sent.

Intercepting Emails

Interceptors allow you to make modifications to emails before they are handed off to the delivery agents. An interceptor class must implement the ::delivering_email(message) method which will be called before the email is sent.

class SandboxEmailInterceptor
  def self.delivering_email(message)
    message.to = ['sandbox@example.com']
  end
end

Before the interceptor can do its job you need to register it using the interceptors config option. You can do this in an initializer file like config/initializers/mail_interceptors.rb:

Rails.application.configure do
  if Rails.env.staging?
    config.action_mailer.interceptors = %w[SandboxEmailInterceptor]
  end
end

NOTE: The example above uses a custom environment called “staging” for a production-like server but for testing purposes. You can read Creating Rails Environments for more information about custom Rails environments.

Observing Emails

Observers give you access to the email message after it has been sent. An observer class must implement the :delivered_email(message) method, which will be called after the email is sent.

class EmailDeliveryObserver
  def self.delivered_email(message)
    EmailDelivery.log(message)
  end
end

Similar to interceptors, you must register observers using the observers config option. You can do this in an initializer file like config/initializers/mail_observers.rb:

Rails.application.configure do
  config.action_mailer.observers = %w[EmailDeliveryObserver]
end