Open mail relay

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An open mail relay is an SMTP server configured in such a way that it allows anyone on the Internet to send e-mail through it, not just mail destined to or originating from known users.[1][2][3] This used to be the default configuration in many mail servers; indeed, it was the way the Internet was initially set up, but open mail relays have become unpopular due to their exploitation by spammers and worms. Many relays were closed, or were placed on blacklists by other servers.


History and technology

Until the 1990s being an open relay was a common configuration for a mail server and was often the default on UNIX systems at installation.[1] This was due, in part, to the traditional store-and-forward method of getting e-mail to its destination. E-mail was passed from computer to computer (through and beyond the Internet) via modems on telephone lines. For many early networks, such as UUCPNET, FidoNet and BITNET, lists of machines that were open relays were a core part of those networks.[2] Filtering and speed of e-mail delivery were not priorities at that time and in any case the government and educational servers with which the Internet was started were covered by a federal edict forbidding the transfer of commercial messages.[4][5]

Abuse by spammers

In the mid-1990s, with the rise of spamming, spammers resorted to re-routing their e-mail through third party e-mail servers to avoid detection[6] and to exploit the additional resources of these open relay servers. Spammers would send one e-mail to the open relay and (effectively) include a large blind carbon copy list, then the open relay would relay that spam to the entire list.[7] While this greatly reduced the bandwidth requirements for spammers at a time when Internet connections were limited, it forced each spam to be an exact copy and thus easier to detect. After abuse by spammers became widespread, operating an open relay came to be frowned upon among the majority of Internet server administrators and other prominent users,[6]. Open relays are recommended against in RFC 2505 and RFC 5321 (which defines SMTP). The exact copy nature of spam using open relays made it easy to create bulk e-mail detection systems such as Vipul's Razor and the Distributed Checksum Clearinghouse. To counter this, spammers were forced to switch to using hash busters to make them less effective and the advantage of using open relays was removed since every copy of spam was "unique" and had to be sent individually.

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