Открытые реле SMTP: история, злоупотребления и меры безопасности.

An open mail relay is a Simple Mail Transfer Protocol (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. 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 because of their exploitation by spammers and worms. Many relays were closed, or were placed on blacklists by other servers.

Until the 1990s, mail servers were commonly intentionally configured as open relays; in fact, this was frequently the installation default setting.

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 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. This trend reduced the percentage of mail senders that were open relays from over 90% down to well under 1% over several years. This led spammers to adopt other techniques, such as the use of botnets of zombie computers to send spam. One consequence of the new unacceptability of open relays was an inconvenience for some end users and certain Internet service providers. To allow customers to use their e mail addresses at Internet locations other than the company's systems (such as at school or work), many mail sites explicitly allowed open relaying so that customers could send e mail via the ISP from any location. Once open relay became unacceptable because of abuse (and unusable because of blocking of open relays), ISPs and other sites had to adopt new protocols to allow remote users to send mail. These include smart hosts, SMTP AUTH, POP before SMTP, and the use of virtual private networks (VPNs). The Internet Engineering Task Force (IETF) has written a best current practices covering Email Submission Operations in RFC 5068. Note that the above only becomes an issue if the user wishes to (or has to) continue to send e mail remotely, using the same SMTP server which they were previously accessing locally. If they have valid access to some other SMTP server from their new, remote location, then they will typically be able to use that new server to send e mails as if from their old address, even when this server is properly secured. (Although this may involve some reconfiguration of the user's email client which may not be entirely straightforward.) The CAN SPAM Act of 2003 makes it illegal to send spam through an open relay in the United States, but makes no provision on their use for personal e mail or their operation in general; the effectiveness of the act has been questioned.

The most famous open mail relay operating today is probably that of John Gilmore, who argues that running an open relay is a freedom of speech issue. His server is included on many open relay blacklists (many of which are generated by "automatic detection", that is, by anti spam blacklisters sending an (unsolicited) test e mail to other servers to see if they will be relayed). These measures cause much of his outgoing e mail to be blocked. John Gilmore and other open relay proponents declare that they do not support spam and spamming, but see bigger threat in attempts to limit Web capabilities that may block evolution of the new, next generation technologies. They compare the network communication restrictions with restrictions that some phone companies tried to place on their lines in the past, preventing transferring of computer data rather than speech.