A basic mailer API intended to be used as a contact form backend for a static site.

config.ru

require 'bundler'
Bundler.require :default

module Mailer
  class Configuration
    attr_accessor :recipient
  end

  def self.configure
    yield config
  end

  def self.config
    @config ||= Configuration.new
  end

  class Request
    REQUIRED_PARAMS = ['name', 'email', 'message']

    attr_reader :errors

    def initialize(env)
      @request = ::Rack::Request.new(env)
      @errors = []
    end

    def valid?
      missing = REQUIRED_PARAMS.select { |p| request.params[p].to_s == '' }

      if missing.any?
        @errors << "Missing #{missing.join(', ')}"
        false
      else
        true
      end
    end

    REQUIRED_PARAMS.each do |name|
      define_method(name) do
        @request.params[name]
      end
    end

    private

    attr_reader :request
  end

  class Email
    attr_reader :to, :from, :name, :message

    def initialize(from, name, message)
      @from = from
      @name = name
      @message = message
    end

    def send!
      mail = Mail.new
      mail.from = from
      mail.to = Mailer.config.recipient
      mail.header['Reply-To'] = "#{name} <#{from}>"
      mail.subject = subject
      mail.body = body

      mail.deliver!
    end

    private

    def subject
      "Submission from #{name} via Mailer"
    end

    def body
      "From: #{name} <#{from}>\n\n#{message}\n\nEmail sent via Mailer"
    end
  end

  class Rack
    def self.call(env)
      request = Mailer::Request.new(env)

      if request.valid?
        Email.new(request.email, request.name, request.message).send!

        response 200, message: 'Message sent'
      else
        response 422, message: request.errors.join(', ')
      end

      rescue Exception => err
        response 500, message: err.message
    end

    private

    def self.response(status, body_hash)
      body = body_hash.to_json
      [status, {'Content-Type' => 'application/json', 'Content-Length' => body.size.to_s}, [body]]
    end
  end
end

Mailer.configure do |config|
  config.recipient = ENV['MAILER_RECIPIENT']
end

run Mailer::Rack

Gemfile

source 'https://rubygems.org'

gem 'rack'
gem 'mail'
gem 'puma'

Gemfile.lock

GEM
  remote: https://rubygems.org/
  specs:
    mail (2.6.3)
      mime-types (>= 1.16, < 3)
    mime-types (2.6.2)
    puma (2.14.0)
    rack (1.6.4)

PLATFORMS
  ruby

DEPENDENCIES
  mail
  puma
  rack

BUNDLED WITH
   1.10.6