Kerio has a variety of features available for mitigating the evil spam gremlins. These include:
1 – SpamAssassin – Open Source spam filter
2 – Directory Harvest Attack Protection – track email coming in for non-existent users and limit the number allowed per host
3 – Policies – tag emails with X-Spam headers, then use local policies, etc. Also write custom filters that identify certain keywords as spam
4 – RBL – A standard with mail servers, Realtime BlackList servers mark common spammers or hosts that do not meet a minimum criteria for being acceptable mail servers
5 – SPF – Rely on srv reccords from domains to specify what IPs are allowed to send mail as a domain
6 – Domain Name Verification – Seems simple enough, don’t accept mail if the domain doesn’t exist
7 – Connection throttling – Don’t allow more than a certain number of sockets to be opened from a given host
8 – SMTP whitelist and blacklist – always accept or always deny mail from a given host that can be defined in the Kerio Admin
9 – Microsoft Caller ID: http://download.microsoft.com/download/2/e/2/2e2850b8-2747-4394-a5a9-d06b5b9b1a4c/callerid_email.pdf
10 – Spam Repellent – delay the SMTP handshake process to deter spammers