In contrast to other more extensive approaches to deal with unsolicited email, commonly called "spam", this memo discusses a very simple authentication scheme. It uses marking of hosts in reverse DNS (in-addr.arpa zone) to allow the receiving mail transfer agents to decide whether the connecting (sending) host is a designated mail transfer agent (MTA) or not.

Despite being a weaker scheme than most of the other proposals currently discussed, it can reduce the amount of spam and viruses/worms significantly and has the advantage that it can be implemented based on existing and well-established Internet technology like DNS without any changes to that technology.