require 'resolv'

class Domain

  def mxers(domain)
    mxs = Resolv::DNS.open do |dns|
      ress = dns.getresources(domain, Resolv::DNS::Resource::IN::MX)
      ress.map { |r| [r.exchange.to_s, IPSocket::getaddress(r.exchange.to_s), r.preference] }
    end
    return mxs
  end

end

p Domain.new.mxers('yahoo.com')
#=> [["mta7.am0.yahoodns.net", "66.94.237.139", 1], ["mta5.am0.yahoodns.net", "67.195.168.230", 1], ["mta6.am0.yahoodns.net", "98.139.54.60", 1]]