A simple MTA Written in ruby.

send_mail.rb

require 'resolv'
require 'net/smtp'
require 'dkim'

Dkim::domain = 'domain.com'
Dkim::selector = 'mail'
Dkim::private_key = <<DKIM_KEY
-----BEGIN RSA PRIVATE KEY-----
Your Private Key Here.
-----END RSA PRIVATE KEY-----
DKIM_KEY

def get_mx_server(email)
  domain = email.split('@')[1]
  mx_records = Resolv::DNS.open do |dns|
    dns.getresources(domain, Resolv::DNS::Resource::IN::MX)
  end
  mx_records.shuffle[0].exchange.to_s
end

def get_response_code(msg)
  msg.split(' ')[0]
end

def send_mail(from, to, msg)
  mx_server = get_mx_server to
  Net::SMTP.start(mx_server, 25, Dkim::domain) do |smtp|
    begin
      res = smtp.send_message Dkim.sign(msg), from, to
			res.status
		rescue Net::SMTPFatalError => e
			get_response_code e.to_s
		end
	end
end