About attacks that can't be prevented by using iptables

To restrict client access on Postfix, change main.cf and add check_client_access to smtpd_client_restrictions as follows:

Ex) smtpd_client_restrictions= 
　permit_mynetworks,
　reject_unknown_client
      :::
  check_client_access hash:/etc/postfix/reject_list ←adding
　permit

Then create the file /etc/postfix/reject_list to describe the IP or domain name that you want to deny in the form of "address REJECT".

Ex)
185.234.217.194 REJECT
spam.example.com REJECT

Hash DB and reload to reflect.

#postmap/etc/postfix/reject_list
# postfix reload

There are many Postfix settings, so please read the documentation carefully and think about the appropriate security settings.