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The "plus addressing" technique appends a password to the "username"
part of the email address.
Cost-based systems
Since spam occurs primarily because it is so cheap to send, a proposed
set of solutions require that senders pay some cost in order to send
spam, making it uneconomic.
Stamps
Some gatekeeper such as Microsoft would sell electronic stamps, and
keep the proceeds. Or a Micropayment, such as Electronic money would
be paid by the sender to the recipient or their ISP, or some other
gatekeeper.
Hashcash
Hashcash and similar systems require that a sender pay a computational
cost by performing a calculation that the receiver can later verify.
Verification must be much faster than performing the calculation, so
that the computation slows down a sender but does not significantly
impact a receiver. The point is to slow down machines that send most
of spam -- often millions and millions of them. While every user that
wants to send email to a moderate number of recipients suffers just a
seconds' delay, sending millions of emails would take an unaffordable
amount of time.
Bonds
As a refinement to stamp systems was the idea of requiring that the
micropayment only be retained if the recipient considered the email to
be abusive. This addressed the principal objection to stamp systems:
popular free legitimate mailing list hosts would be unable to continue
to provide their services if they had to pay postage for every message
they sent out.
Issues
A difficulty that must be dealt with by most anti-spam methods,
including DNSBLs, Authentication and Reputation (A&R),
Sender-supported whitelists and tags, Ham passwords, cost-based
systems, Heuristic filtering, and Challenge/response systems is that
spammers already (illegally) use other people's computers to send
spam. The computers in question are already infected with viruses and
spyware operated by the spam senders, in some cases seriously damaging
the computer's responsiveness to the legitimate user. Spam from the
legitimate user's computer can be sent using the user's and/or
system's identity, list of correspondents, reputation, credentials,
stamps, hashcash and/or bonds. The added motivation to steal from such
systems in order to abuse these things may simply impel spammers to
infect more computers and cause greater damage. On the other hand,
this could compel computer users to finally secure their systems,
reducing Botnets, which would have myriad other benefits, as they are
used for extortion, phishing, and terorrism, as well as spam.
Ultimately, any system that holds senders responsible for the mail
they send needs to deal with the situation of irresponsible senders
that may send both spam and ham.
Heuristic filtering
Heuristic filtering, such as is implemented in the program
SpamAssassin, uses some or all of the various tests for spam mentioned
above, and assigns a numerical score to each test. Each message is
scanned for these patterns, and the applicable scores tallied up. If
the total is above a fixed value, the message is rejected or flagged
as spam. By ensuring that no single spam test by itself can flag a
message as spam, the false positive rate can be greatly reduced.
Tarpits and Honeypots
A tarpit is any server software which intentionally responds
pathologically slowly to client commands. A honeypot is a server which
attempts to attract attacks. Some mail administrators operate tarpits
to impede spammers' attempts at sending messages, and honeypots to
detect the activity of spammers. By running a tarpit which appears to
be an open mail relay, or which treats acceptable mail normally and
known spam slowly, a site can slow down the rate at which spammers can
inject messages into the mail facility.
One tarpit design is the teergrube, whose name is simply German for "tarpit."
This is an ordinary SMTP server which intentionally responds very
slowly to commands. Such a system will bog down SMTP client software,
as further commands cannot be sent until the server acknowledges the
earlier ones. Several SMTP MTAs, including Postfix and Exim, have a
teergrube capacity built-in: when confronted with a client session
which causes errors such as spam rejections, they will slow down their
responding [4]. A similar approach is taken by TarProxy.
Another design for tarpits directly controls the TCP/IP protocol
stack, holding the spammer's network socket open without allowing any
traffic over it. By reducing the TCP window size to zero, but
continuing to acknowledge packets, the spammer's process may be tied
up indefinitely. This design is more difficult to implement than the
former. Aside from anti-spam purposes, it has also been used to absorb
attacks from network worms. [5]
