This would have to be done within the actual email program that you are using. For example, if you were using Postfix, here is what you would do:

Go to /etc/postfix/main.cf. You will see a reference to the smtpd_recipient_restrictions section and it looks at check_client_access cidr:/etc/postfix/cidr_greylist_network_exceptions.

Adding the IP, xxx.xxx.xxx.0/24 OK and reloading Postfix allows mail from that IP through.