I have postfix on my server for handling mail but postfix does not read hosts.deny. Instead I have this in my main.cf:
smtpd_client_restrictions =
check_client_access hash:/etc/postfix/client_access
and made a script that changes all the "ALL: ip.add.re.ss" into "ip.add.re.ss REJECT" in my /etc/postfix/client_access and executes "postmap /etc/postfix/client_access".
But to take things a bit further, scanning for ip-adresses in mail.log which attempts to use relaying could be even nicer...