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Distributed Server Boycott List (DSBL)

It receives relay messages and publishes the results online. DSBL never sends messages.

The Tester

This could be anybody who has the DSBL test software or a compatible program installed. The recommended way to run a tester is to feed spam into the spamtrap program (or operate a spamtrap and let spammers feed it), this will test all the hosts that have sent spam. Of course each tester is free to determine his/her own test policy.
Relay test message in order to see if a host that transferred the spam was a vulnerable open relay or proxy the software run by the tester emulates what the spammers do and tries to relay a test message to DSBL, using the various ways spammers use.


Before doing a test the tester asks DSBL for a unique string, which is used as the test identification. DSBL uses this cookie to determine whether the relay message comes from a trusted user or not.

Open relay

Email host that allows anybody to send messages anywhere, often only using obscure SMTP syntax. These hosts are often abused by spammers to get their messages sent out for free.

Open proxy

Proxy server or firewall that allows anybody to connect anywhere, often used by spammers to obscure their point of origin, also used in combination with open relays.
How are hosts tested?
  • The tester asks dsbl for a unique cookie, giving username/password if the tester has them.
  • DSBL returns a cookie and remembers the cookie in the database, noting down whether the cookie was for a trusted user or not.
  • The tester runs relay tests on the host.
  • If the test message is relayed to DSBL, a server will be listed in the DSBL database

In which list is a server listed?

There are 3 lists, here all single hop relays that were tested by trusted testers get listed., the outputs of multihop relays tested by trusted testers get listed here., the output servers of tests by untrusted and anonymous users get listed here.
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