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Spam Filters To Prevent Both Outbound & Inbound Spams

Spam filters detect unsolicited, unwanted, and virus-infested email (called spam) and stop it from getting into email inboxes. Internet Service Providers (ISPs) use spam filters to make sure they aren’t distributing spam. Small- to medium-sized businesses (SMBs) also use spam filters to protect their employees and networks.

Spam filters are applied to both inbound email (email entering the network) and outbound email (email leaving the network). ISPs use both methods to protect their customers. SMBs typically focus on inbound filters.

Spam Filters

At the very least, spam email is a nuisance that will clog up your employees’ inboxes and overload your servers. Spam is also dangerous — the entry point for serious attacks that could damage your computers, your computer network, your bottom line, and even your company’s reputation. Yes, you need a spam filter solution as the key first line of defense.

It is estimated that 70 percent of all email sent globally is spam, and the volume of spam continues to grow because spam remains a lucrative business. Spammers get ever more sophisticated and creative in their tactics to get their messages into your inboxes and wreak their havoc.

Why are Spam filters important?

Important to realize, there are many spam filtering solutions available. But, spam filtering solutions cannot be 100 percent effective. However, a business email system without spam filtering is highly vulnerable, if not unusable.

It is important to stop as much spam as you can, to protect your network from the many possible risks: viruses, phishing attacks, compromised web links, and other malicious content. Spam filters also protect your servers from being overloaded with non-essential emails. And even the worse problem of being infected with spam software that may turn them into spam servers themselves.

By preventing spam email from reaching your employees’ mailboxes, spam filters give an additional layer of protection to your users, your network, and your business. When you decide to invest or upgrade your spam filter solution, know that there are countless spam filter programs out there. And it will take time to figure out which one works best for your business.

Here are a few key things to look for:

  • At a minimum, the solution you choose must block spam. This might sound like a given, but not all spam filter software is up to the job (or doesn’t keep up to date with the evolving world of spam attacks).
  • The solution you choose must provide the security you need for your network, but not stop the legitimate emails your employees need to conduct their business.
  • Administrators must have the ability to edit and create rules over and above predefined rule settings. So that the solution meets your organizational needs. However, this customization should be easy, even for unsophisticated computer users.

Attackers can send volumetric DDoS traffic to overwhelm your services, degrade network performance, or bring down end-user machines individually.

Additionally, attackers can also take control of end-user resources or steal credentials. Read More Here: Spam Filtering | What is It and How Does It Operate?

How do Spam filters work?

In the world of spam prevention, IP addresses are like social security numbers. The IP address is the only reliable identifying information that email receivers can use to identify the responsible sender of each message.

Spam filters use “heuristics” methods, which means that each email message is subjected to thousands of predefined rules (algorithms). Each rule assigns a numerical score to the probability of the message being spam, and if the score passes a certain threshold the email is flagged as spam and blocked from going further.

There are different types of spam filters for different criteria:

  • Content filters – parse the content of messages, scanning for words that are commonly used in spam emails.
  • Header filters – examine the email header source to look for suspicious information (such as spammer email addresses).
  • Blacklist filters – stop emails that come from a blacklist of suspicious IP addresses. Some filters go further and check the IP reputation of the IP address.
  • Rules-based filters – apply customized rules designed by the organization to exclude emails from specific senders, or emails containing specific words in their subject line or body.

You can also run your message through certain tools, such as Mail Tester to test the ‘spammyness’ of an email.

No single method is a complete solution to the spam problem, and there are always trade-offs (which the heuristics try to weigh) between rejecting legitimate email vs. letting spam slip through. With this in mind, please Read More about Spam Filtering Methods | 10 Techniques You Should Know.

Outbound & Inbound spam filtering

Service providers and enterprises need to pay attention to the spam exiting their network in the outbound direction. If left unchecked, outbound spam will cause the service provider’s network to be blocked by the rest of the internet. Effectively, stopping legitimate email traffic for all network customers.

This happens as part of the global effort to stop inbound spam from harming users. Sophisticated reputation systems block IP addresses that send too much spam, and rate limits those with questionable sending practices, by blacklisting them.

Spam Filters

On one hand, Outbound spam filtering addresses the challenge of IP blacklisting. While on the other hand, Inbound spam filtering is the act of filtering an email that is sent to your inbox to ensure it is not spam. Given that the number of spam emails sent worldwide every day far exceeds the number of legitimate emails sent.

Generally, inbound spam filtering helps to keep our inboxes safe, secure and manageable.

How does Outbound spam filtering work?

When multiple users send email through one mail server, email receivers on the internet can’t trust anything other than the IP address of the mail server. Simply, because spammers can provide fake email addresses or even impersonate legitimate users.

Outbound spam filtering involves more than just analyzing message content and rejecting the spam. A good outbound spam filter knows how to identify the actual sender of each message, and to record the long-term behavior of each sender, looking for suspicious patterns of behavior.

A good outbound spam filter also takes great care not to make mistakes, because mistakes hurt your own users, rather than someone else’s.

How does Inbound spam filtering work?

Fundamentally, every spam filter is a classifier that attempts to accurately guess whether email recipients will consider a piece of email to be spam. Inbound spam filtering typically identifies the sender by the IP address from which the message originated.

Modern inbound spam filters combine inputs from a variety of machine and human-crafted logic to classify messages. Some systems use a technique called locality-sensitive hashing, which reduces each message down to a simple numerical representation. Such that two email messages that are similar will have a similar representation.

Others apply a large set of human-crafted rules, or heuristics, which attempt to identify aspects of the message. That either increases or decreases the probability that the message will be seen as spam by a recipient.

Managing and tracking sender identity in an inbound spam control system is relatively easy. Because IP addresses are nearly impossible to falsify (in the context of SMTP email). Today, inbound spam filtering is more about protecting end-users from cybercrime than it is about removing annoying or unwanted messages.


Most email service providers offer inbound spam filtering to detect and isolate spam email before it gets to user inboxes. Stopping spam delivery helps increase productivity because it eliminates wasted time dealing with unwanted messages and worse, malicious network attacks.

Malware from spam emails can cost organizations millions of dollars for removal and data recovery. Unfortunately, just a single click on a malicious link can infect your network. Inbound spam filtering lessens the chance of attack by reducing your overall exposure to spam.

By tracking individual users, the outbound spam filter can identify spam-like behavior on a user-by-user basis, and prevent spam from leaking out of the mail server’s IP address.

Resourceful References;

I hope you have gathered enough information in regards to the above-revised guide about Spam Filtering Methods. But, if you have additional information, contributions or even suggestions, please Contact Us.

You can also share some or more of your thoughts in the comments box below this post. Below are more additional and related to the topic links.

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