Seguir la siguiente documentacion:

• https://www.howtoforge.com/hardening-postfix-for-ispconfig-3
• https://www.linuxbabe.com/mail-server/block-email-spam-postfix

No configurar greylist en su lugar configurar postscreen

• https://www.altraforma.com/linux/como-activar-postfix-postscreen-en-linux-para-filtrar-spam/
• https://www.howtoforge.com/community/threads/solved-postfix-mail-anti-spam-security.78512/#post-372919

Llenar la configuración del postscreen

• https://git.ispconfig.org/ispconfig/ispconfig3/issues/4239

Instalar postwhite

• https://github.com/stevejenkins/postwhite
• https://github.com/spf-tools/spf-tools