ActionMailer Setup

Mailer Class:

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

# app/mailers/notification_mailer.rb
class NotificationMailer < ApplicationMailer
  def welcome_email(user)
    @user = user
    @login_url = login_url
    mail(to: user.email, subject: "Welcome to Our App")
  end

  def password_reset(user)
    @user = user
    @reset_url = password_reset_url(user.reset_token)
    mail(to: user.email, subject: "Password Reset Instructions")
  end
end

HTML Template:

<%# app/views/notification_mailer/welcome_email.html.erb %>
<h1>Welcome, <%= @user.name %>!</h1>
<p>Thanks for signing up. Get started by logging in:</p>
<%= link_to "Login Now", @login_url, class: "button" %>

Text Template:

<%# app/views/notification_mailer/welcome_email.text.erb %>
Welcome, <%= @user.name %>!

Thanks for signing up. Get started by logging in:
<%= @login_url %>

Usage (Async with SolidQueue):

# In controller or service
NotificationMailer.welcome_email(@user).deliver_later
NotificationMailer.password_reset(@user).deliver_later(queue: :mailers)

Why: ActionMailer integrates seamlessly with SolidQueue for async delivery. Always use deliver_later to avoid blocking requests. Provide both HTML and text versions for compatibility.

# ❌ WRONG - Blocks HTTP request thread
def create
  @user = User.create!(user_params)
  NotificationMailer.welcome_email(@user).deliver_now  # Blocks!
  redirect_to @user
end

# ✅ CORRECT - Async delivery via SolidQueue
def create
  @user = User.create!(user_params)
  NotificationMailer.welcome_email(@user).deliver_later  # Non-blocking
  redirect_to @user
end

Why bad: deliver_now blocks the HTTP request until SMTP completes, creating slow response times and poor user experience. deliver_later uses SolidQueue to send email in background.

class NotificationMailer < ApplicationMailer
  def custom_notification
    @user = params[:user]
    @message = params[:message]
    mail(to: @user.email, subject: params[:subject])
  end
end

# Usage
NotificationMailer.with(
  user: user,
  message: "Update available",
  subject: "System Alert"
).custom_notification.deliver_later

Why: Cleaner syntax, easier to read and modify, and works seamlessly with background jobs.

Email Templates

HTML Layout:

<%# app/views/layouts/mailer.html.erb %>
<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <style>
      body {
        font-family: Arial, sans-serif;
        max-width: 600px;
        margin: 0 auto;
        color: #333;
      }
      .header {
        background-color: #4F46E5;
        color: white;
        padding: 20px;
        text-align: center;
      }
      .content {
        padding: 20px;
      }
      .button {
        display: inline-block;
        padding: 12px 24px;
        background-color: #4F46E5;
        color: white;
        text-decoration: none;
        border-radius: 4px;
      }
      .footer {
        padding: 20px;
        text-align: center;
        font-size: 12px;
        color: #666;
      }
    </style>
  </head>
  <body>
    <div class="header">
      <h1>Your App</h1>
    </div>
    <div class="content">
      <%= yield %>
    </div>
    <div class="footer">
      <p>&copy; 2025 Your Company. All rights reserved.</p>
    </div>
  </body>
</html>

Text Layout:

<%# app/views/layouts/mailer.text.erb %>
================================================================================
YOUR APP
================================================================================

<%= yield %>

--------------------------------------------------------------------------------
© 2025 Your Company. All rights reserved.

Why: Consistent branding across all emails. Inline CSS ensures styling works across email clients.

class ReportMailer < ApplicationMailer
  def monthly_report(user, data)
    @user = user

    # Regular attachment
    attachments["report.pdf"] = {
      mime_type: "application/pdf",
      content: generate_pdf(data)
    }

    # Inline attachment (for embedding in email body)
    attachments.inline["logo.png"] = File.read(
      Rails.root.join("app/assets/images/logo.png")
    )

    mail(to: user.email, subject: "Monthly Report")
  end
end

In template:

<%# Reference inline attachment %>
<%= image_tag attachments["logo.png"].url %>

Why: Attach reports, exports, or inline images. Inline attachments can be referenced in email body with image_tag.

# ❌ WRONG - Relative path doesn't work in emails
def welcome_email(user)
  @user = user
  @login_url = login_path  # => "/login" (relative path)
  mail(to: user.email, subject: "Welcome")
end

# ✅ CORRECT - Full URL works in emails
def welcome_email(user)
  @user = user
  @login_url = login_url  # => "https://example.com/login" (absolute URL)
  mail(to: user.email, subject: "Welcome")
end

# Required configuration
# config/environments/production.rb
config.action_mailer.default_url_options = { host: "example.com", protocol: "https" }

Why bad: Emails are viewed outside your application context, so relative paths don't work. Always use *_url helpers to generate absolute URLs.

Email Testing

Configuration:

# Gemfile
group :development do
  gem "letter_opener"
end

# config/environments/development.rb
config.action_mailer.delivery_method = :letter_opener
config.action_mailer.default_url_options = { host: "localhost", port: 3000 }

# config/environments/production.rb
config.action_mailer.delivery_method = :smtp
config.action_mailer.smtp_settings = {
  address: "smtp.sendgrid.net",
  port: 587,
  user_name: Rails.application.credentials.dig(:smtp, :username),
  password: Rails.application.credentials.dig(:smtp, :password),
  authentication: :plain,
  enable_starttls_auto: true
}
config.action_mailer.default_url_options = { host: "example.com", protocol: "https" }

Why: letter_opener opens emails in browser during development - no SMTP setup needed. Test email appearance without actually sending.

# test/mailers/previews/notification_mailer_preview.rb
class NotificationMailerPreview < ActionMailer::Preview
  # Preview at http://localhost:3000/rails/mailers/notification_mailer/welcome_email
  def welcome_email
    user = User.first || User.new(name: "Test User", email: "test@example.com")
    NotificationMailer.welcome_email(user)
  end

  def password_reset
    user = User.first || User.new(name: "Test User", email: "test@example.com")
    user.reset_token = "sample_token_123"
    NotificationMailer.password_reset(user)
  end

  # Preview with different data
  def welcome_email_long_name
    user = User.new(name: "Christopher Alexander Montgomery III", email: "long@example.com")
    NotificationMailer.welcome_email(user)
  end
end

Why: Mailer previews at /rails/mailers let you see all email variations without sending. Test different edge cases (long names, missing data, etc.).

# test/mailers/notification_mailer_test.rb
class NotificationMailerTest < ActionMailer::TestCase
  test "welcome_email sends with correct attributes" do
    user = users(:alice)
    email = NotificationMailer.welcome_email(user)

    # Test delivery
    assert_emails 1 do
      email.deliver_now
    end

    # Test attributes
    assert_equal [user.email], email.to
    assert_equal ["noreply@example.com"], email.from
    assert_equal "Welcome to Our App", email.subject

    # Test content
    assert_includes email.html_part.body.to_s, user.name
    assert_includes email.text_part.body.to_s, user.name
    assert_includes email.html_part.body.to_s, "Login Now"
  end

  test "delivers via background job" do
    user = users(:alice)

    assert_enqueued_with(job: ActionMailer::MailDeliveryJob, queue: "mailers") do
      NotificationMailer.welcome_email(user).deliver_later(queue: :mailers)
    end
  end

  test "password_reset includes reset link" do
    user = users(:alice)
    user.update!(reset_token: "test_token_123")
    email = NotificationMailer.password_reset(user)

    assert_includes email.html_part.body.to_s, "test_token_123"
    assert_includes email.html_part.body.to_s, "password_reset"
  end
end

Why: Test email delivery, content, and background job enqueuing. Verify recipients, subjects, and that emails are queued properly.

Email Configuration

Development:

# config/environments/development.rb
config.action_mailer.delivery_method = :letter_opener
config.action_mailer.perform_deliveries = true
config.action_mailer.raise_delivery_errors = true
config.action_mailer.default_url_options = { host: "localhost", port: 3000 }

Test:

# config/environments/test.rb
config.action_mailer.delivery_method = :test
config.action_mailer.default_url_options = { host: "example.com" }

Production:

# config/environments/production.rb
config.action_mailer.delivery_method = :smtp
config.action_mailer.perform_deliveries = true
config.action_mailer.raise_delivery_errors = false
config.action_mailer.default_url_options = {
  host: ENV["APP_HOST"],
  protocol: "https"
}

config.action_mailer.smtp_settings = {
  address: ENV["SMTP_ADDRESS"],
  port: ENV["SMTP_PORT"],
  user_name: Rails.application.credentials.dig(:smtp, :username),
  password: Rails.application.credentials.dig(:smtp, :password),
  authentication: :plain,
  enable_starttls_auto: true
}

Why: Different configurations per environment. Development previews in browser, test stores emails in memory, production sends via SMTP.

# test/mailers/notification_mailer_test.rb
class NotificationMailerTest < ActionMailer::TestCase
  setup do
    @user = users(:alice)
  end

  test "welcome_email" do
    email = NotificationMailer.welcome_email(@user)

    assert_emails 1 { email.deliver_now }
    assert_equal [@user.email], email.to
    assert_equal ["noreply@example.com"], email.from
    assert_match @user.name, email.html_part.body.to_s
    assert_match @user.name, email.text_part.body.to_s
  end

  test "enqueues for async delivery" do
    assert_enqueued_with(job: ActionMailer::MailDeliveryJob) do
      NotificationMailer.welcome_email(@user).deliver_later
    end
  end

  test "uses correct queue" do
    assert_enqueued_with(job: ActionMailer::MailDeliveryJob, queue: "mailers") do
      NotificationMailer.welcome_email(@user).deliver_later(queue: :mailers)
    end
  end
end

# test/system/email_delivery_test.rb
class EmailDeliveryTest < ApplicationSystemTestCase
  test "sends welcome email after signup" do
    visit signup_path
    fill_in "Email", with: "new@example.com"
    fill_in "Password", with: "password"
    click_button "Sign Up"

    assert_enqueued_emails 1
    perform_enqueued_jobs

    email = ActionMailer::Base.deliveries.last
    assert_equal ["new@example.com"], email.to
    assert_match "Welcome", email.subject
  end
end

Official Documentation:

Rails Guides - Action Mailer Basics

Gems & Libraries:

letter_opener - Preview emails in browser during development

Tools:

Email on Acid - Email testing across clients

Email Service Providers:

SendGrid Rails Guide

rails-ai:mailers

Email with ActionMailer