Email list MX record checker

email_list_mx_checker.rb

# Read stdin, extract email addresses
# Look up MX records for those addresses
# Return each email address and their first MX record
# (or IP address, as a fallback)

hosts = {}

STDIN.each_line do |l|
  # Extract the email and the email domain from each line
  email = l[/^([^\s]+\@[^\s+\,]+)\b/, 1]
  host = email[/\@(.+)$/, 1]

  # If already cached, use what was cached
  if hosts[host]
    dns = hosts[host]
  else
    # Do an MX lookup
    mx = `dig MX +short #{host}`
    dns = if mx =~ /[a-z]+/i
      mx.downcase
    else
      # Technically mail can still work if the host
      # has an A record
      `dig A +short #{host}` =~ /[a-z]+/i ? a : 'FAIL'
    end

    # Just get the first entry
    dns = dns.split("\n")[0]
    hosts[host] = dns
  end
  puts "#{email},#{dns}"
end

I only built this for my own use and rate limiting on DNS could be an issue depending on your setup. However, the memoization helps a lot, and you can add @ entries to the dig calls if you need to route the DNS elsewhere. Google's 8.8.8.8 is usually good for hundreds of QPS.