Ruby on Rails

# Rails credentials are the idiomatic place. This opens an editor:
bin/rails credentials:edit

# Add:
#   emailssh_api_key: esh_your_key_here
# The key comes from https://emails.sh/dashboard/api-keys.

# lib/emailssh_delivery.rb
require 'net/http'
require 'json'
require 'uri'

class EmailsshDelivery
  ENDPOINT = URI('https://emails.sh/v1/emails').freeze

  def initialize(settings)
    @key = settings.fetch(:api_key)
  end

  # ActionMailer hands us a Mail::Message. Pull out the parts the API wants.
  def deliver!(message)
    payload = {
      from: message[:from].formatted.first,
      to: Array(message.to),
      subject: message.subject,
      html: body_of(message, 'text/html'),
      text: body_of(message, 'text/plain')
    }.compact
    payload[:cc] = Array(message.cc) if message.cc
    payload[:reply_to] = Array(message.reply_to).first if message.reply_to

    request = Net::HTTP::Post.new(ENDPOINT)
    request['Authorization'] = "Bearer #{@key}"
    request['Content-Type'] = 'application/json'
    request.body = JSON.generate(payload)

    response = Net::HTTP.start(ENDPOINT.host, ENDPOINT.port, use_ssl: true, read_timeout: 10) do |http|
      http.request(request)
    end

    return JSON.parse(response.body) if response.is_a?(Net::HTTPSuccess)

    refused = JSON.parse(response.body)['error']
    raise "emails.sh refused the send: #{refused['code']}: #{refused['message']} #{refused['next']}"
  end

  private

  def body_of(message, mime_type)
    return message.body.decoded if message.mime_type == mime_type

    message.find_first_mime_type(mime_type)&.decoded
  end
end

# config/initializers/emailssh.rb
require Rails.root.join('lib/emailssh_delivery')

ActionMailer::Base.add_delivery_method(
  :emailssh,
  EmailsshDelivery,
  api_key: Rails.application.credentials.emailssh_api_key
)

# config/environments/production.rb
config.action_mailer.delivery_method = :emailssh
config.action_mailer.default_options = { from: 'Acme <hello@acme.com>' }
config.action_mailer.default_url_options = { host: 'acme.com', protocol: 'https' }

class OrderMailer < ApplicationMailer
  def shipped(order)
    @order = order
    mail(to: order.customer_email, subject: "Order #{order.number} has shipped")
  end
end

# In the controller or the job, unchanged from whatever you had before.
OrderMailer.shipped(order).deliver_later