Send email in Ruby on Rails with ActionMailer

class InvoiceMailer < ApplicationMailer
  def receipt
    @order = params[:order]

    attachments["invoice-#{@order.number}.pdf"] = @order.invoice_pdf   # binary string
    attachments["terms.pdf"] = File.read(Rails.root.join("public/terms.pdf"))
    attachments.inline["logo.png"] = File.read(Rails.root.join("app/assets/images/logo.png"))
    # in the view: <%= image_tag attachments["logo.png"].url %>

    headers["X-PostStack-Tags"] = "receipt,transactional"
    headers["X-PostStack-Idempotency-Key"] = "receipt-#{@order.id}"

    mail(
      to: @order.email,
      cc: "accounts@example.com",
      reply_to: "billing@yourdomain.com",
      subject: "Receipt for order ##{@order.number}"
    )
  end
end

# config/credentials/production.yml.enc (decrypted view)
poststack:
  api_key: sk_live_...

# config/environments/production.rb
config.action_mailer.smtp_settings = {
  address:              "smtp.poststack.dev",
  port:                 587,
  user_name:            "poststack",
  password:             Rails.application.credentials.dig(:poststack, :api_key),
  authentication:       :plain,
  enable_starttls_auto: true
}

# test/mailers/previews/user_mailer_preview.rb
class UserMailerPreview < ActionMailer::Preview
  def welcome
    UserMailer.with(user: User.first).welcome
  end
end

# open http://localhost:3000/rails/mailers/user_mailer/welcome

# config/environments/development.rb
config.action_mailer.delivery_method = :letter_opener   # gem "letter_opener"
config.action_mailer.perform_deliveries = true

# config/environments/test.rb (Rails default)
config.action_mailer.delivery_method = :test

# test/mailers/user_mailer_test.rb
class UserMailerTest < ActionMailer::TestCase
  test "welcome" do
    user = users(:alice)
    email = UserMailer.with(user: user).welcome
    assert_emails(1) { email.deliver_now }
    assert_equal ["hello@yourdomain.com"], email.from
    assert_equal "Welcome to Acme", email.subject
  end
end

# staging: same SMTP settings as production, with an sk_test_ key

require "net/http"
require "json"
require "securerandom"

class Poststack
  URI_EMAILS = URI("https://api.poststack.dev/emails")

  def self.send_email(payload, idempotency_key: SecureRandom.uuid, attempts: 3)
    attempts.times do |attempt|
      res = Net::HTTP.start(URI_EMAILS.host, URI_EMAILS.port, use_ssl: true,
                            open_timeout: 5, read_timeout: 10) do |http|
        req = Net::HTTP::Post.new(URI_EMAILS)
        req["Authorization"]   = "Bearer #{ENV.fetch('POSTSTACK_API_KEY')}"
        req["Content-Type"]    = "application/json"
        req["Idempotency-Key"] = idempotency_key   # same key on every retry
        req.body = payload.to_json
        http.request(req)
      end

      case res.code.to_i
      when 200..299 then return JSON.parse(res.body)   # => {"id"=>"em_..."}
      when 429, 500..599
        sleep(res["retry-after"]&.to_i || 2**attempt)
      else
        # 400 invalid payload, 401/403 key problem, 422 unverified domain / suppressed
        raise "PostStack #{res.code}: #{res.body} (request #{res['x-request-id']})"
      end
    end
    raise "PostStack: retries exhausted"
  end
end

Poststack.send_email({   # braces: a positional hash, not keyword arguments
  from: "Acme <hello@yourdomain.com>",
  to: ["user@example.com"],
  subject: "Hello from Ruby!",
  html: "<h1>Welcome!</h1>",
  text: "Welcome!"
})

# gem install mail   (or add gem "mail" to your Gemfile)
require "mail"

Mail.defaults do
  delivery_method :smtp,
    address:              "smtp.poststack.dev",
    port:                 587,
    user_name:            "poststack",
    password:             ENV.fetch("POSTSTACK_API_KEY"),
    authentication:       :plain,
    enable_starttls_auto: true
end

Mail.deliver do
  from     "Acme <hello@yourdomain.com>"
  to       "user@example.com"
  subject  "Hello from Ruby!"

  text_part do
    body "Welcome! Thanks for signing up."
  end
  html_part do
    content_type "text/html; charset=UTF-8"
    body "<h1>Welcome!</h1><p>Thanks for signing up.</p>"
  end

  add_file "./invoice.pdf"   # optional attachment
end