Skip to content

Email Prioritization

Reducing delays on legitimate mail caused by junk mail

Email Prioritization is a server-level anti-spam technique developed at HP Labs, Bristol. It was first proposed in a technical report, and presented at the Usenix Annual Technical Conference in Boston, Massachusetts.

How does it work?

Spam not only clogs inboxes and wastes users’ time; it often slows the delivery of legitimate e-mail due to the sheer volume of junk passing through corporate servers. As a result, some companies could be forced to invest in more robust hardware to keep mail flowing at acceptable speeds.

But there’s a less expensive way to speed legitimate messages through the system: by classifying them as probable “good” or “junk” mail before they’re sent to be scanned.

Servers tend to be “faithful”. In other words, if a server sent a good message before, it’s likely to send a good one next time, too; if it sent junk before, chances are high that it’s sending junk again. “New” servers that have no history of sending mail to your system before are probably sending spam.

Servers can detect the sending IP address from a message header, before the full message is scanned for viruses or spam content and sent for delivery, so those likely to be deemed “good” can go to the head of the queue for processing.

(from PCWorld article HP Labs Develops Spam ID System)

What’s in it for me?

Faster delivery of legitimate mail. Increased server-level resilience to spam/viral attack; even when your mail server is getting bombarded by bad mail, your good mail is still going to get through.


Two versions of Email Prioritization were developed during the research; one based on sendmail and the other on postfix.

Postfix has many advantages over sendmail, particularly when implementing the Email Prioritization scheme. If possible, we would strongly recommend using postfix.

Having been published at USENIX, Email Prioritization is in the public domain, and any developer if free to write code based on the ideas presented.

When I have done so, some example code should appear here;

In the meantime, please feel free to read some articles.

Articles discussing Email Prioritization

Email Prioritization: reducing delays on legitimate mail caused by junk mail

The original technical report, published by HP Labs, Bristol.

HP Labs Develops Spam ID System

PCWorld article discussing the Email Prioritization following the presentation at USENIX.

Email Prioritization and Greylisting

It’s possible to combine Email Prioritization and Greylisting, allowing mail servers to more accurately Greylist junk mail whilst letting the good mail through.

About me

My name is Dan Twining, and I’m one of the researchers who worked on Email Prioritization at HP Labs. Email Prioritization is a great idea, but like many ideas developed in such large research institutes, Email Prioritization was unable to make it out of the lab and become part of HP’s core product offering.

I’ve since left HP Labs, and I set up these web pages as a more publicly visible and accessible record of our work, in the hope that I could encourage sysadmins and developers to make use of our ideas.

No comments yet

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: