Many vendors offer solutions that sound like traffic shaping to block unknown spammers but most offer rate limiting rather than true traffic shaping.
Rate limiting restricts the number of messages a sender can send in a particular time frame or the number of recipients each message can have. In the early days this worked for high volume senders but it is ineffective against today's botnets that can trickle messages from many unique server IP's.
Prior to traffic shaping the most effective mechanism for turning away unknown servers was greylisting. When Greylisting detects messages from unknown senders it sends a tempfail message to the sending server requesting the server to try again later. It then remembers the addresses and if it sees the message again, delivers the message.
Unfortunately this technique for edge anti-spam control has dwindled in its effectiveness since its creation in 2003. Greylisting used to be effective at getting rid of 50% of spam traffic; now it is less than 10% effective as spammers have learned to immediately resend their messages.
Greylisting has failed because spammers have written more intelligent sending agents.
Traffic shaping is the logical replacement for greylisting, because spammers cannot avoid it by simply trying to send a second time.
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