List of notes:
[1] https://barredowlweb.com/2016/10/25/fighting-spam-block-entire-ttld-with-postfix/
[2] https://www.linuxbabe.com/mail-server/block-email-spam-postfix
[3] https://www.exratione.com/2019/02/a-mailserver-on-ubuntu-18-04-postfix-dovecot-mysql/
[4] https://krebsonsecurity.com/2018/06/bad-men-at-work-please-dont-click/
[5] https://bobcares.com/blog/fail2ban-postfix-sasl/
[6] https://blog.matrixpost.net/email-authentication-by-using-dmarc-dkim-and-spf/
[7] https://www.interserver.net/tips/kb/tips-to-block-email-spam-with-postfix-smtp-server/