Why forge and how to protect yourself from someone else sending emails from my domain?

Question

HTML

Good day toasters and toasters!
Essence: there is a domain moydomen.ru. There is traffic to it: about 30,000-40,000 visitors per day. The site provides free intermediary services. In its niche, it ranks second in Russia. There is paid advertising, and of course there is a mass mailing of HTML-letters by email to site users from
[email protected] - something about bitcoins. Access to the mail is carried out through mail.ru password hacking was not, MiTM too.
And, accordingly, two questions: why fake my domain and write about bitcoins? (I know about the elementary substitution of headers in php) but for the purpose of mass mailing?
And second: how can this be banned?

Answer 1

I will add to previous answers.

• if you use SPF, then with ~all, not -all (the latter is often recommended in articles), otherwise you will have problems with forwarding your letters.
  
• DKIM and DMARC are mandatory. SPF with a strict prohibition of sending from other hosts without DKIM will only make it worse, and without it it simply does not work.