One of our e-mail filtration systems uses several of what
are called RBL's (Realtime Blackhole List) to stop messages
before they even enter HomerNet's e-mail system. RBL lists
are databases which store information (IP addresses, latest
spam reports, etc) on spammers who operate on the Internet.
The HomerNet mail system looks at the IP address any server
that tries to deliver a message to one of our users. It takes
the IP address (which is one of the only parts of an e-mail
message that cannot be forged) of the sending server and looks
it up in several different RBL lists:
http://www.spamcop.net/bl.shtml
SpamCop is very agressive and it's database is being
constantly updated thanks to anonymous spam reports.
http://www.ordb.org/lookup
Open Relay Database [ORDB] only lists open relays which
are usually e-mail systems for legitimate companies,
but are not configured correctly to disallow unauthorized
mailing.
http://dnsbl.njabl.org
NJABL.org (Not Just Another Black List) lists open relays,
open proxies, open form to mail HTTP gateways,
dynamic IP pools, and direct spammers.
If a match is found in any of these RBL's, the message will
be refused by HomerNet's e-mail system. If a no match is found,
it is allowed in and delivered to the appropriate user's mailbox.
It's important to remember that there can be false positives
using this type of filtering. It is possible for non-spam to
be blocked if someone is sending a message through a system
that has been listed for sending large amounts of spam out
onto the Internet. However, any message that is blocked from
coming into our system, whether it's because the sending server
is listed in one of the above RBL's or for any other reason,
the message IS NOT simply lost. HomerNet's system will tell
the sender why the message has been blocked and where to go
to view the listing details. The sending e-mail system will
then bounce the message back to the original sender.
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