If you wish to restrict SMTP access via IP addresses or a network range, one way to do it would be via Postfix configuration. You can set up restrictions using a Postfix hash-table or via a cidr file. You can choose one or both methods to perform the restrictions. The end-result will be the same.
Create a text file with the addresses you would like to reject. You can choose whatever file name you wish. E.g.: /etc/postfix/smtp_client_access