Until the email from the sender reaches the end user, it is faced with many filters that affect deliverability and entering the inbox.


Anti-spam gateway filters: These are physical servers that are included in the system of companies and can be considered as the first assistants of ISPs in the line of defense. Every email that comes to the company has to pass this filter before it reaches the inbox. The only disadvantage of these filters, which learn over time which email is spam and which is harmless, is that they cannot develop by learning different types of spam from the outside world because they only examine the emails that come to the company.


3rd party spam filters: These are filters by different companies that try to separate harmless emails from spam emails with content control and certain reputation criteria in line with their own methods. 3rd party filters can be involved in the decision-making processes of gateway filters, as well as have an effect on whether the emails that pass this filter belong to the spam folder or the inbox. Due to the large number of companies using these filters, they learn a lot about spam email. This provides high information about which email will be processed.


Desktop spam filters: It is the protection method available on the end user’s computer. Perhaps the most difficult spam filters to pass, as they can be adjusted to personal preferences. The most known of these is the Outlook program that uses Microsoft’s SmartScreen spam filter.


The basis of the filter technology


Spam filter technology can be used in both “inbound” (emails entering the system) and “outbound” (emails being sent from the system) stages. ISPs generally prefer both methods to protect users. Although the transmitters are exposed to both filters, the “inbound” method is usually the protection method that causes concern. Either way, thousands of predefined algorithms work to determine where an email belongs.


The Email Filtering Methodology


ISPs examine filtering under 3 main headings. These:

  • Sending source of the email
  • Sender’s reputation
  • The content of the email sent
The Source


Spam senders try to defeat the reputation system by changing the IP and domain every time. However, spam filters perform the filtering process by looking at the sender verification, continuity and how long sending was actualized from the same IP and domain. ISPs always approach emails sent by new IP and domains with extra caution. Senders using the same IP and domain for a long time tend to have a stronger reputation and are considered more reliable.




The sender’s reputation is calculated as a result of algorithms and millions of data analysis. The reputation score is graded as 0 to 100 (Return Path) or -10 to 10 (IronPort). ISPs perform the filtering process of the incoming e-mail according to the reputation score. The main reputation titles are as follows:

  • Spam complaints
  • Spam trap addresses
  • Message content
  • Volume
  • Blacklists

Since it is perhaps the most important criterion in determining the reputation score, keeping the complaint rates low and adjusting the frequency of sending with quality content should be the main goal of every email marketer.




Perhaps every element of the email is checked during content analysis. “Header”, “footer” structure, layout of HTML coding, image sizes used, color and size of fonts in the content, URL lengths, subject title, attachment and many other items are the titles in which the content is viewed while passing through the filter.


To sum up, spam filters are an important part of the email ecosystem. Without them, millions of spam e-mails would invade the servers, almost preventing the system from working. We can call them guarding friends. We are pleased that they prevent unwanted emails from falling into our inbox, as well as delivering the messages we need to receive. And again, thanks to the filters, important transactional emails reach us.


In the new article of our series, we will examine the effect of IP address and domain on deliverability.


Previously on this guide:

The Essential Email Deliverability Guide Part 1 – Know More About Spam And Spam Filters