Home / Blogs

Greylisting Still Works - Part I

John Levine

Greylisting is a hoary technique for rejecting spam sent by botnets and other poorly written spamware. When a mail server receives an attempt to deliver mail from a hitherto unseen sending host IP address, it rejects the message with a "soft fail" error which tells the sender to try again later. Real mail software does try again, at which point you note that the host knows how to retry and you don't greylist mail from that IP again. The theory is that spamware doesn't retry, so you won't get that spam. I wrote a paper on it for the 2005 CEAS conference, and concluded that conservative greylisters worked well.

We've now been using greylisting for close to a decade, and some people have argued that it's no longer useful, since the bad guys could easily fix their spamware to retry, or since bots are so cheap, they could just send everything twice. So does it still work?

I recently went through my greylister's logs and collected some statistics for both a recent week, and the past year, about hosts that I greylisted:

WeekYear
No retry12121294812
One retry745662402
Many messages495674590

The first row is the number of hosts that got a soft fail and never came back. The second row is the number that retried the message that failed, but never sent anything again, and the third row is the number that retried and sent more messages after that.

As you can see, for the week, about half of the greylisted hosts didn't retry, and over a year, about 2/3 didn't. That's still a lot of mail my mail server didn't have to filter. I attribute the different ratios to the shutdown of several botnets over the past year, evidently botnets that didn't retry.

So it's certainly not a magic bullet (what is?) but greylisting still is an effective way to deter a lot of spam cheaply.

Next, Greylisting Still Works - Part II

By John Levine, Author, Consultant & Speaker. More blog posts from John Levine can also be read here.

Related topics: Email, Malware, Security, Spam

 
   
WEEKLY WRAP — Get CircleID's Weekly Summary Report by Email:

Comments

To post comments, please login or create an account.

Related Blogs

Related News

Explore Topics

Industry Updates – Sponsored Posts

How Savvy DDoS Attackers Are Using DNSSEC Against Us

Facilitating a Trusted Web Space for Financial Service Professionals

MarkMonitor Partners with CYREN to Deepen Visibility into Global Phishing Attacks

Verisign Named to the Online Trust Alliance's 2016 Honor Roll

Port25 Announces Release of PowerMTA V4.5r5

New Case Study: Jobtome.com Replaces 30 Postfix Servers with a Single PowerMTA

Verisign Q1 2016 DDoS Trends: Attack Activity Increases 111 Percent Year Over Year

Is Your TLD Threat Mitigation Strategy up to Scratch?

i2Coalition to Host First Ever Smarter Internet Forum

An Update on Port25 and the Future of PowerMTA - One Year Later​

Encrypting Inbound and Outbound Email Connections with PowerMTA

Resilient Cybersecurity: Dealing with On-Premise, Cloud-Based and Hybrid Security Complexities

V12 Group Sustains Customer Satisfaction by Deploying PowerMTA for Launchpad Platform

Verisign Releases Q4 2015 DDoS Trends - DDoS Attack Activity Increasing by 85% Year Over Year

Best Practices from Verizon - Proactively Mitigating Emerging Fraudulent Activities

Neustar Data Identifies Most Popular Times of Year for DDoS Attacks in 2015

The Framework for Resilient Cybersecurity (Webinar)

2015 Trends: Multi-channel, Streaming Media and the Growth of Fraud

PowerMTA Now Offers Scheduled Delivery Control

Data Volumes and Network Stress to Be Top IoT Concerns

Sponsored Topics

Port25

Email

Sponsored by
Port25
Verisign

Security

Sponsored by
Verisign
Afilias

DNS Security

Sponsored by
Afilias
Afilias - Mobile & Web Services

Mobile

Sponsored by
Afilias - Mobile & Web Services