How Do Email Spam Filters Work?

Unsolicited commercial email (UCE) is the digital junk mail known as spam. At the very least UCE is a nuisance, and at its worst an access point for viruses and malicious code. However you may view it, spam is an ever-present problem for most businesses and individuals.

Considering that the volume of email worldwide is 269 billion messages per day and that 49.7% of it is spam, you can understand the need for a good email spam filter.

email spam filter

{photo credit: flickr}

But how does an email spam filter work?

A filtering solution applied to your email system uses a set of protocols to determine which of your incoming messages are spam and which are not. There are several different types of spam filters available:

  • Content filters – review the content within a message to determine if it is spam or not
  • Header filters – review the email header in search of falsified information
  • General blacklist filters – stop all emails that come from a blacklisted file of known spammers
  • Rules-based filters – use user-defined criteria – such as specific senders or specific wording in the subject line or body – to block spam
  • Permission filters – require anyone sending a message to be pre-approved by the recipient
  • Challenge-response filters – require anyone sending a message to enter a code in order to gain permission to send email

Spam filtering solutions are commonly deployed 3 different ways – hosted or in the “cloud”, on-premise appliance such as a Barracuda Spam Filter, and software installed on PCs that integrate with an email client such as Microsoft Outlook. Although no spam filtering solution is 100% effective, a business email system without spam filtering is virtually unusable. Many spam emails contain infected email attachments that contain viruses, phishing attacks, compromised web links and other malicious content. By preventing them from reaching your mailbox, your spam filter offers an additional layer of protection to your users.

How to Pick a Good Spam Filter

The spam filter you choose for your business use depends largely on the amount of email you and your employees receive daily. It also depends on the types of emails sent and received, and your company’s needs and preferences. A business that needs tight security controls may opt for a permission-based filter, while a content filter may be more useful for a business whose employees receive articles and newsletters via email.

Spam is a multi-billion dollar business, and it isn’t going away. If anything, spammers will continue to improve their tactics to get their messages into your inbox, where they can wreak havoc on your network. Choose a spam filter that will provide the level of security you need for your network while still allowing your employees to conduct business via email.

[Editor’s update: This article was originally written in November of 2013 when the worldwide email sent was 144.8 billion messages per day and 65% of it was spam. As of June, 2017, that number is up to 269 billion messages per day and 49.7% is spam. That just shows that this problem isn’t going away anytime soon. ]

How much email spam does your company receive?

  • Bruce Nuffer

    Regarding spam, I’ve long had a single question. We have a sophisticated spam filter where I work, and it even flags my own test emails as spam. Yet there are a couple actual spam emails I get regularly that make it past the filters despite breaking nearly every example commonly listed. They regularly include dollar signs and exclamation points in the subject line, include text that says ON SALE! and SHOP NOW, and have unsubscribe info (that doesn’t actually work). How do they get past the filters?? They aren’t embedding this info in images, this is actual text.

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