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How Traffic Shaping Differs from Greylisting and Rate Limiting

By Desmond Liao | 2 minute read

Many hosting companies use anti-spam techniques that sound like traffic shaping to pre-filter spammers but what they are actually doing is rate limiting. Rate limiting restricts the quantity of messages a sender can send in a particular time frame or the number of recipients each message can have. A few years ago, this worked for high volume senders but it has become less effective against today’s botnets that can trickle messages from many unique server IP’s.

Until about 2005, a simple anti-spam technique called “greylisting” provided a very effective defense against spam. “Greylisting” means temporarily rejecting connections from new senders, forcing them to retry message delivery a number of times before their messages are accepted. The idea is that legitimate senders will retry (as required by the SMTP standard), whereas spammers will not (because their SMTP servers are often simplistic and non standards-compliant).

Grey-listing was initially very effective at getting rid of spam. But with time, spammers wrote better software that was able to retry message delivery, getting spam through the greylisting barrier and into end users’ mailboxes. Whereas greylisting was 50% effective in 2005, our latest statistics show that greylisting now gets rid of less than 10% of spam traffic.

Unfortunately this technique for controlling high spam traffic has become less effective since its creation in 2003 because spammers have simply written more intelligent spamming software.

On the other hand, spammers cannot avoid traffic shaping by simply trying to send an email a second time. Successfully implementing traffic shaping requires more than simply inserting delays in an SMTP session.

Traffic shaping selectively restricts the bandwidth for untrusted senders to only a few bytes per second. High quality senders are expedited to get mail through to you even more quickly, while spammers are placed in a holding pattern. Instead of a spammer clogging your network and slowing down SMTP responses, it’s the spammer who gets bogged down. The result is a clean mail stream of less than 30% its original volume.

Traffic Shaping is absolutely free for non-commercial users (less than 10,000 SMTP connections per day):

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