How is the spam filter set up?

Question

Mail server

I came across SPF but hardly apply only the checks described there.

Answer 1

SpamAssassin, for example, puts an email through a series of tests, each of which contributes positively or negatively to the final score. At the end, the resulting score is compared with the specified boundary value, and based on the result of the comparison, a decision is made whether it is spam or not spam.
The tests include DKIM/SPF/DMARC, search for the sender's address and links from the email in the spammers' database, and checks for general parameters - mismatches (or absence) of the PLAIN part of the email with the HTML part, the ratio of the volume of images and text, the correctness of the encoding headers and body of the letter, etc., etc., etc.