From Digibahn: 550Flush is a tool that can end incoming spam to your mail server. It leverages existing commands in SMTP (reference RFC 821) to enforce whitelist based e-mail delivery to your mail server. This approach allows your server to receive only e-mail from senders who are on your permitted list (the whitelist). The premise is that there is a relatively finite amount of senders from which you actually want to receive e-mail.
read more +