How do I stop spam emails from our domain email addresses?

Question

Mail server

Our email addresses on the domain send spam emails to customers.
Also sometimes rollbacks come from unaccepted letters that we did not send (to addresses unknown to us).
If I understand correctly, there are only three options:

• spam bot sends letters not from our mail server, but simply forges the signature of the sender - resp. do I need to check the headers of these spam emails? Another question is how the spam bot found out our addresses, they are not published on the site and the addresses of our customers are also natural!
  
• spam bot connects to our mail server - but we changed passwords! where else could he get them?
  
• the virus on the computer sends spam - but we checked with antivirus programs and more than once - everything is clean
  
• on the site, the viral code sends out mailings - but where does it get the addresses of our customers, we have no base on the site
  
• the mail server is hacked on the server - but is that even possible?
  

what else? where to look? how to act?

Answer 1

Take one particular spam email and carefully read the title, you will immediately understand where it comes from. In 95% of cases, your address was simply forged, it's not difficult.
What to do, first set up an spf record for your domain and server. Then you can do dkim as well. This will partially solve the problem but will not remove it completely.
There has never been a 100% solution and never will be.

Answer 2

Open mail relay - forwarding through your server.
Close the mailer settings.

Answer 3

Write to hosters where the donem is parked and indicate the articles of the law on spam from 100t to 500t fine. It’s a pity they don’t give enough for such srachs, I would give at least 3 years so that these bastards of spammers would be less at the end, and the hosters also fined the hosts with a trailer for this, they turned the Internet into a sewer!