## 1 Introduction  
Email has become an extremely important resource for communication because it greatly facilitates the exchange of information. The first attempt to build a mechanism similar to modern email (electronic mail) was made in 1965 at the Massachusetts Institute of Technology (MIT) as part of a university file-sharing system that could be accessed from remote terminals. Later, the first email standard was proposed in 1973 by the US DARPA (Defense Advanced Research Projects Agency). It was then introduced in Arpanet in 1977, already including fields such as \"From\" and \"To\" and functionalities such as forwarding emails to other users. As expected, with the widespread use of email for both personal and professional purposes, the proliferation of practices such as spamming and even the sending of malicious emails also began. The first documented case of the commercial practice of spam occurred in April 1994 (available at [[https://www.wired.com/1999/04/the-spam-that-started-it-all/](https://www.wired.com/1999/04/the-spam-that-started-it-all/)]([https://www.wired.com/1999/04/the-spam-that-started-it-all/](https://www.wired.com/1999/04/the-spam-that-started-it-all/)), accessed on 1 September 2022). By the late 1990s, spam had already become a major problem, as more and more advertisers made use of it as a zero-cost advertising practice that allows for wide dissemination. Thus, in 2002, the European Union launched the \"Directive on privacy and electronic communications (available at [[https://eur-lex.europa.eu/legal-content/ES/ALL/?uri=CELEX%3A32002L0058](https://eur-lex.europa.eu/legal-content/ES/ALL/?uri=CELEX%3A32002L0058)]([https://eur-lex.europa.eu/legal-content/ES/ALL/?uri=CELEX%3A32002L0058](https://eur-lex.europa.eu/legal-content/ES/ALL/?uri=CELEX%3A32002L0058)), accessed on 1 September 2022). This directive includes a specific section on spam, which makes it illegal to send unsolicited commercial communications without the receiver's prior permission. However,the legal measures have not been particularly effective. According to Statista (available at [[https://www.statista.com/statistics/420400/spam-email-traffic-share-annual/](https://www.statista.com/statistics/420400/spam-email-traffic-share-annual/)]([https://www.statista.com/statistics/420400/spam-email-traffic-share-annual/](https://www.statista.com/statistics/420400/spam-email-traffic-share-annual/)), accessed on 1 September 2022), there has been a considerable reduction in spam traffic in recent years, from 66.76% in 2014 to 45.56% in 2021. This decrease has been possible thanks to hardware-based anti-spam solutions (e.g., Cisco's Email Security Appliance, available at [[https://www.cisco.com/c/es_es/products/security/email-security/index.html](https://www.cisco.com/c/es_es/products/security/email-security/index.html)]([https://www.cisco.com/c/es_es/products/security/email-security/index.html](https://www.cisco.com/c/es_es/products/security/email-security/index.html)), accessed on 1 September 2022), software-based solutions (e.g., ZeroSpam, SpamAssassin, etc. cited in the document available at [[https://www.getastra.com/blog/knowledge-base/best-spam-filters-for-emails/](https://www.getastra.com/blog/knowledge-base/best-spam-filters-for-emails/)]([https://www.getastra.com/blog/knowledge-base/best-spam-filters-for-emails/](https://www.getastra.com/blog/knowledge-base/best-spam-filters-for-emails/)), accessed on 1 September 2022), anti-spam firewalls, or spam filtering services included in email servers. Many of the current spam filtering techniques were not developed and applied until the beginning of the 21 st century.