SpamAssassin VS OSSEC

spamassassin or rspamd or ASSP. All very good. Spamassassin just works, but you need to feed it a good cohort of spam and ham (about 1000 messages each). Most people only send it the spam which will then bias it quite a bit and not have it classify spam as spam unless it's the worst of the bunch. Spamassassin is a filter for your mail server. rspamd is the same idea. ASSP is highly configurable and a proxy.

To detect whether incoming emails are malicious, mail servers also use spam scanners, such as Apache's popular SpamAssasin. The internal workings of these spam scanners can be somewhat complicated (involving Naive Bayes Classifiers on trained, large datasets, for the curious), but the primary takeaway is that these classification systems typically assign a numerical point value to an incoming email to determine the validity of the message. The higher the score, the more likely that the email is spam. For reference, the ISP Optimum states the following regarding their spam filtering:

OSSEC: https://www.ossec.net/ HIDS

It's been a long time since I tried using OSSEC but maybe that would help. It's a Host-based IDS, rather than a network based IDS like Snort. Last time I checked you could point it towards your logs and it will parse them offline similar to how Snort can read a PCAP file.

Not sure if it could be considered one but maybe OSSEC Host Intrusion Detection System might get you some.of the way can be found here

WAZUH (fork of OSSEC would be my first choice when it comes to Linux based HIDS (host based), and Snort or Suricata if you are looking for NIDS (network based). As well as Lynis for ensuring the setup of the host is as you intended.

cnhids is a Host Intrusion Detection System for cardano node based on https://github.com/ossec/ossec-hids: