This option works only if you have rebuilt the mail server in cwp.admin with antispam.
You can blacklist or whitelist incoming emails in the following files:

/etc/postfix/sender_blacklist
/etc/postfix/sender_whitelist

After making changes you need to run the following command for the files you edited

postmap /etc/postfix/sender_blacklist
postmap /etc/postfix/sender_whitelist

Options:
# Example for blacklisting domains/IP's or emails, one per line:

1.2.3.4 REJECT
1.2.3.5 REJECT
mail.freemailer.tld REJECT We don't allow emails from mail.freemailer.tld
test@test.test REJECT

After REJECT you can add the message which will be returned to the sender.
The whitelist is the same only difference is that instead of REJECT you use OK.

Example for whitelisting domains/IP's, one per line:

1.2.3.4 OK
1.2.3.5 OK
mail.freemailer.tld OK
test@test.test OK