What is an ISP Spam Filter and How Does It Work?


Each day, our societies and communities become increasingly connected via the evolution of the internet and its online capabilities.  With technological advancements paving the way for new means of communication, research, business and entertainment, the possibilities are seemingly endless.

However, when discussing the capabilities of the internet, it is also crucial to acknowledge the associated risks and problems. Alongside the wealth of potential are dangerous opportunities for those attempting to commit spam, fraud and gain the personal information of others.

One of the most common threats to internet users comes in the form of email spam, sent to unsuspecting accounts in the hopes of gaining information or committing some form of fraud. Internet service providers (ISP) are constantly trying to monitor and prevent spam from reaching their users, however this is no small task and requires additional software.

Here is our guide to ISP spam filters and the ways in which they operate.

What Is ISP Spam?

Essentially, ISP Spam refers to any kind of unwanted content directed at users’ mailboxes.  This could be anything from junk mail and hoax advertising, to content with more sinister intentions, such as malware attacks and fraud attempts. In the case of most spam attacks, the primary goal is to extract users’ personal information, usually in the hopes of monetary gain.

As mentioned above, ISP and hosts are always blocking and deflecting spam attempts but, to be done properly and efficiently, this requires additional technology and software. This is where ISP spam filters come into play, monitoring, identifying and blocking unwanted emails from reaching users’ mailboxes.

How Do ISP Spam Filters Operate?

So how do these filters work? Essentially, ISP spam filters utilise finely programmed algorithms to sift through email messages being sent and received within an internet service provider and identify the messages that present spam-like characteristics. At this point, the filter calculates the chances of the message being malicious or spam and weighs up the total risk against a predetermined threshold. If the email is deemed too likely to be spam, it will be flagged and blocked from entering the main mailbox of the user.

Not all spam filters are the same and, depending on the type of filter, there are a few different criteria that may be used to figure out if a message is spam or not. The elements of an email that are scanned may include:


As simple as it may sound, the header of an email alone can be used to determine if the message is spam or not. By looking at an email header, a spam filter can identify suspicious email addresses, misspelt words and other spam-like qualities.

Body Content

A spam filter will also sift through the content of an email, analysing it for suspicious phrases, file names and attachments. As well as what is contained in an email, spam filters also assess the formatting of messages, focussing on elements such as font use, text size variations and images.

Blacklist Filtering

Additionally, one of the many ways that ISP filters identify spam is by comparing the IP addresses and domain names of senders to existing blacklists. Many spam filters record the information of senders reported as spam by users, allowing them to cross examine incoming mail against pre-existing records and data.


Another way that spam filters sift through messages is by using permission filters. These require the recipient to have previously agreed to receive content from the sender. If a user has not agreed to receive emails from the address, any messages will be prevented from entering their mailbox.

Rule-Based Filtering

As mentioned previously in this article, many spam filters utilise predetermined rules or algorithms when identifying unwanted mail. These rules may include words or phrases decided by the user or ISP. When the filter identifies potential spam features, it assigns a point score to the email. If the email totals a certain number of points, the filter will presume it is spam and block it.

What Do Filters Prevent?

When it comes to the safety and security of internet users, there are a multitude of threats to consider and prevent. As an internet service provider, spam filters are an essential means of safeguarding the information of users and preventing the spread of malicious software and attachments.

At their simplest, spam filters prevent unwanted emails from entering mailboxes but, in doing so, they can prevent serious issues from arising such as data leaks and bank fraud. The intention of spammers and fraudsters is usually to gain money, personal details or render a device useless so it is crucial their efforts are thwarted.

Common Types Of Email Spam

When it comes to spam mail, there are numerous different ways in which fraudsters may attempt to deceive recipients and convince them of an email’s legitimacy. Often, emails will be disguised as real messages, made to seem as if they contain important information.

As technology and spam filters have advanced, so have the techniques adopted by scammers. For this reason, it is crucial that internet service providers use a thorough and reliable spam filter to protect their users and information. With that being said, here are a few common types of spam mail.

Phishing & Fake Advertising

The first, and most common, form of spam mail comes in the form of phishing emails. Phishing attacks consist of a spammer impersonating a real person or business. Within these types of attack, fraudsters will often try to convince the recipient that they have legitimate intentions and that you can benefit from what they have to offer.

Frequently, phishing mail will act as if it is a legitimate service and ask for personal details such as usernames, passwords, bank details and account numbers. Spotting these types of email often requires a bit of awareness and common sense, but it isn’t always that simple. Therefore, spam filters are crucial for the prevention of these sorts of mail and the protection of users.

Baiting & Hoaxes

Next on our list are hoaxes and baiting attempts. Similarly to phishing mail, baiting involves a fraudster trying to convince an individual that they have won some form of reward or are eligible for an exclusive offer. The scammer will usually claim this is a time sensitive opportunity in order to convince the user to act quickly and without better judgement.

These emails are particularly dangerous for older people and unsuspecting internet users as they often seem to present incredible offers. However, as is often the case online, if it seems too good to be true, it probably is.

Malicious Content

Finally, another method used by scammers to trick users is to send an email containing what seems to be a legitimate link or attachment, encouraging individuals to open or download it.

Unfortunately, these emails usually contain some form of virus, malware or ransomware. Once downloaded or opened, the user’s computer will be infected with software that will either obtain their personal information, or render the machine useless until a ransom fee of some sort is paid.

Anti Spam For ISP Email Providers

When it comes to the safety and security of users, ISP spam filters are a crucial element for any provider. Not only do they protect individuals, they also protect the reputation of ISPs.

Here at MailCleaner, our spam filters are favoured by a number of ISPs and internet email providers  for a multitude of reasons. Not only are our filters incredibly user friendly, but they also offer multi-domain configuration, mail routing, SMTP callouts and parallel filter capabilities.

What’s more, all of our spam tools, including cloud hosting and software installation, are fully scalable. This gives you complete control and the ability to protect any number of mailboxes, from 10 to 50,000.


No matter the size, or requirements of your organisation, look no further than MailCleaner for spam protection and security. As an open source solution, we are favoured by ISPs all over the world.

Get in touch with us today to discuss your specifications with a member of our expert team, we’re always happy to help.