email-mx-check.rb

#!/usr/bin/env ruby
require 'resolv'

file = ARGV[0]

if file.nil? || file == ""
  puts "email-mx-check /path/to/file-with-emails-line-per-line"
  exit
else
  puts "the following domains have a missing mx-entry:"
end

def valid_domain_mx_entry?(e,result=false)
  fqdn = if e.include?("@")
    e.split("@")[1]
  else
    e
  end
  if !fqdn.nil? && fqdn != ""
    mx_records = Resolv::DNS.open(nameserver: ['9.9.9.9']) do |dns|
      dns.getresources(fqdn, Resolv::DNS::Resource::IN::MX)
    end
    result = true if !mx_records.nil? && mx_records != []
  end
  result
end

File.readlines(file, chomp: true).each do |line|
    puts(line) if !line.nil? && line != "" && !valid_domain_mx_entry?(line.downcase)
end

This script checks if there is an mx entry for each email-address listed in the given file and outputs the email-addresses without an mx-entry. Optional you can give list of domains in the file.