Postfix

Limit incoming Mail Rate against Spam Scripts:

edit main.cf and set:

smtpd_error_sleep_time = 1s
smtpd_soft_error_limit = 5
smtpd_hard_error_limit = 10
smtpd_client_connection_count_limit = 10
smtpd_client_connection_rate_limit = 10

Testing Mail Loop (send a Mail every Second):

 
while true; do `date | mail -s "Test Postfix" postbox@yourdomain.com`;sleep 1; done

Result: after 10Mails the sending IP should by blocked! Add "fail2ban" Log Monitor as second wall protection to kick out bad IP's forever!