Custom Rails Newsletter using Postmark

newsletter.rb

# app/models/newsletter.rb

class Newsletter < ApplicationRecord
  has_rich_text :content

  def deliver!
    if Rails.env.production?
      # Postmark bulk API
      client = Postmark::ApiClient.new(Rails.application.credentials.postmark_api_key)
      messages = NewsletterSubscriber.active.map do |subscriber|
        NewsletterMailer.send_newsletter(self, subscriber)
      end
      client.deliver_messages(messages)
    else
      NewsletterSubscriber.active.find_each do |subscriber|
        NewsletterMailer.send_newsletter(self, subscriber).deliver_now
      end
    end

    update(sent_at: Time.current)
  end
end

newsletter_mailer.rb

# app/mailers/newsletter_mailer.rb

class NewsletterMailer < ApplicationMailer
  def send_newsletter(newsletter, subscriber)
    @newsletter = newsletter
    @subscriber = subscriber
    @unsubscribe_token = @subscriber.signed_id

    headers["MESSAGE-STREAM"] = "broadcast"
    mail to: subscriber.email, subject: newsletter.subject
  end
end