Open Mail Relay - Abuse By Spammers

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 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. 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. 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.

Since open mail relays make no effort to verify that the owner of an address is the actual sender of an e-mail, open mail relays are vulnerable to address spoofing.