What is a Spam Filter?

So what is a spam filter and how do they work? Considering that around 270 billion emails and messages speed through the Internet daily and a whopping 50% of this number is spam, you can imagine the kind of tsunami systems are fending off day in and day out. In this context, anti spam measures save system resources by actively blocking spam even before they enter. Let's find out how anti spam work to further appreciate what they do for email accounts on a daily basis.

A spam filter is developed to detect and prevent unwanted and uninvited emails from entering the inbox of the users. Spam filter is programmed to follow a specific set of criteria to accept or deny emails. It checks a particular combination of words or specific phrases in the subject lines and body texts to avoid and prevent them from the user’s inbox. However, this type of filtering of unwanted emails does not work right and there are times when legitimate mails are denied while the real spam mails enters the users’ inbox. There are yet more sophisticated and advanced filters which implements heuristic filters that are more efficient to detect spams through a specific set of word patterns and frequency.

Spam filters are a vital tool as most of emails are spam, therefore a robust spam filter loaded with features is critical to ensure spam-free and clean inboxes. By keeping spam filters, users can exclude infectious spam emails. It enhances the efficiency and avoids manual intervention which is a complex task by itself to filter out the spam messages from legitimate messages.

A spam filter can be maintained as a software routine that would automate the deletion of incoming spam or redirection of identified spam mails to the junk folder.

Spam filtering is performed to detect spam messages based on a specific set of criteria, which comprises of specific phrases, words in the subject line or body text, email addresses or even the attachments that are a part of the mail.

No spam filters are perfect to completely filter out spam emails. There are two prime weaknesses that can impact even the email services that are in demand.

Even the most effective spam filters cannot block all the spam emails

Most of the spam filters categorize the legitimate mails as spam emails and the users miss on the emails as they are moved to junk or spam folders.

How does spam filter work?

Over 80% of the all the email traffic are unsolicited mails that are randomly sent in bulk and these mails are called spam mails. However, these spam mails leave a clue for the spam filters to identify, terminate and redirect them to spam folder.

There are different types of filters that are used to identify the signs and signals of e-mail spams which includes

  • User Defined Filters
  • Header Filters
  • Language Filters

User-defined filters – These type of filters identify spam mails based on subject lines, body text and suspicious attachments and forward the emails to different mailboxes. Users can define or organize their inboxes based on a particular ruleset to forward emails that comes from particular set of friends or any other specifically defined sub-folders. On the other hand the spam emails are filtered in a separate junk folder if the content has originated from malicious source. A database is created to archive the similar traits of the spam emails to help users decide if the emails from a certain set source are unsolicited.

Header filters – Header filters are more clear, in this the email headers are verified and checked it for any possible suspicious mail activities through the information held in subject lines that is presented by the mail browsers. The header filters skim through the information passed on by the servers when delivering mails which are called relay chain. Spammers pass in wrong data in the relay chain to escape detection and to deny people from replying to the emails and to track the location.

Language Filters – It sifts out the emails that does not match the language of the recipient. They are not much in demand as the English language is globally accepted. The current day scenario demands more efficient and robust content filters that can improve the efficiency of spam filtering

Bayesian Analysis – This is a method that relies on a rule-definition to filter out spam mails – the by validating the email context (words and phrases) that could be a sign to indicate spam. The implement a formulae to review and evaluate the content of the mail and do a comparison check with the content that has been defined and archived in the database. This type of filters would help to differentiate valid mails from spam mails over a period of time and the there is an improvement in accuracy.

Collaborative Filtration - A database is created by evaluating and receiving feedback from existing users, which is then used to detect spam. Once the user receives a new spam, it is archived in the blacklist locally or through manual intervention to record the spam characteristics. The records can be used in future to compare with the shared database to comprehend if the email is a spam.

How Anti Spam Block Unsolicited Messages?

Anti spam or spam filters work primarily through several protocols or rules on what to block. This is because spam flood in through different methods and anti spam protocols confront each flood through these particular rules. Each anti spam measure works through these specific filters:

Types of Spam Filters

  1. Blacklist filters
  2. Header filters
  3. Rules-based filters
  4. Challenge-response filters
  5. Content filters
  6.  Permission filters


  • 1. Blacklist filters – Anti spam professionals collect the server addresses of known spammers and creates rules for servers to monitor and label messages coming from these servers. Some ISPs which do not have strong anti spam measures or tolerate spammers within their system tend to have messages from their accounts tagged as spam.
  • 2. Header filters – These anti spam filters work on checking if there is anything remiss on the header information of the message. Each email has its own unique message identifier – spam use only one identifier for a batch of messages. Anti spam filters catch the duplication or see where the message is coming from through the headers and block the message accordingly.
  • 3. Rules-based filters – These anti spam filters tag messages as spam based on specific programmed criteria. For example, if a message has a phrase like "Nigerian Prince" or "viagra," chances are the message will land in the Spam Folder.
  • 4. Challenge-response filters – These anti spam filters work by asking people who want to send a message to enter a code before being able to send you a message.
  • 5. Content filters – These anti spam filters check messages for specific phrases or content which will indicate if it is spam or not.
  • 6. Permission filters – This particular anti spam filter works by asking permission from the receiver if they are willing to accept a message from a particular sender. This in effect, whitelists the email address of the sender if the receiver grants permission.


How Spam Filters are Installed?

These anti spam measures are installed in three ways. Some anti spam filters exist primarily as cloud applications like Comodo Antispam for example. There are anti spam filters which come in the form of hardware. Professional IT techs install these equipment in the premises of the company. Some companies buy licenses for anti spam software and install them along with their email clients like Outlook.

Here's an Example of an Anti Spam Solution That Works

Comodo Antispam's email protection is robust and better than other solutions in the market right now. This email protection is largely due to three layers of proprietary Comodo technology. There's the advanced email protection algorithm, email protection containment technology, and the command console. Here's what Comodo Antispam provides in terms of advantages in line with the ones other cloud services providers offer in terms of email protection:

  • 1. A centralized command console which puts user and group mail management under the fingertips of MSP personnel.

Admins can profile their clients' users and give each user a different security profile. Done this way, companies can better their anti spam protection by applying security features and restrictions on a case per case basis. Administrators will be able to shield the most vulnerable users' email security by doing so.

  • 2. Comodo's unique containment technology works to cage zero-day threats entering cloud services.