SMTP Tarpits
One of the possible avenues that were considered to battle bulk-spam at one time, was to mandate a small fee for every submitted mail. By introducing such artificial cost, with negligible impact on legitimate use as long as the fee is small enough, automated mass-scale spam would instantly become unattractive. Tarpitting could be seen as a similar (but technically much less complex) approach, where the cost for the spammer would be measured in terms of time and efficiency rather than financials.
Authentication procedures increase response times as users attempt invalid passwords. SMTP authentication is no exception. However, server-to-server SMTP transfers, which is where spam is injected, require no authentication. Various methods have been discussed and implemented for SMTP tarpits, systems that plug into the Mail Transfer Agent (MTA, i.e. the mail server software) or sit in front of it as a proxy.
One method increases transfer time for all mails by a few seconds by delaying the initial greeting message ("greet delay"). The idea is that it will not matter if a legitimate mail takes a little longer to deliver, but due to the high volume, it will make a difference for spammers. The downside of this is that mailing lists and other legitimate mass-mailings will have to be explicitly whitelisted or they will suffer too.
Some email systems, such as sendmail 8.13+, implement a stronger form of greet delay. This form pauses when the connection is first established and listens for traffic. If it detects any traffic prior to its own greeting (in violation of RFC 2821) it closes the connection. Since many spammers do not write their SMTP implementations to the specification, this can reduce the number of incoming spam messages.
Another method is to delay only known spammers, e.g. by using a blacklist (see Spamming, DNSBL). OpenBSD has integrated this method into their core system since OpenBSD 3.3, with a special-purpose daemon (spamd) and functionality in the firewall (pf) to redirect known spammers to this tarpit.
MS Exchange can tarpit senders who send to an invalid address. Exchange can do this because the SMTP connector is connected to the authentication system.
A more subtle idea is greylisting, which, in simple terms, rejects the first connection attempt from any previously-unseen IP address. The assumption is that most spammers make only one connection attempt (or a few attempts over a short period of time) to send each message, whereas legitimate mail delivery systems will keep retrying over a longer period. After they retry, they will eventually be allowed in without any further impediments.
Finally, a more elaborate method tries to glue tarpits and filtering software together, by filtering e-mail in realtime, while it is being transmitted, and adding delays to the communication in response to the filter's "spam likeliness" indicator. For example, the spam filter would make a "guess" after each line or after every x bytes received as to how likely this message is going to be spam. The more likely this is, the more the MTA will delay the transmission.
Read more about this topic: Tarpit (networking)