|E-Mail Filtering - Yaritz Consulting: The Consultation Company|
There are many ways that can help to prevent unwanted email or UCE. The best method so far is to use a combination of the following methods. Each method is geared to block in its own way. Using all of these methods, one can get the optimal blocking.
RBLs - Realtime Black Lists
Most RBL list are only good for repeat spammers. It happens to be a good thing since spammers sent their mass mail from one address and then move on. This type of spamming could be considered "war spamming" as a parody on "war dialing". A RBL list works by someone entering an IP or hostname, which is used by others to block their spam. Once blocked the spammer must change to a new address in order to start the process over. Most RBL lists also include a blackhole e-mail address with the purpose of marking all message to it as spam. This will cause that "war spamming" to slow down over time as all the addresses with an ISP will become blocked. More information can be found here.
SPF - Sender Policy Framework
SMTP is a simple protocol, which when used properly, can prevent forged e-mail. Forged e-mail is defined as e-mail that comes from someone who does not "own" a mailbox or e-mail server. SPF solves this problem as the owner of a mail server can enter into DNS all the servers that are able to send e-mail from his domain. This will prevent spammer and other forgers from using an e-mail, but can also cause problems with archaic forwarding. More information can be found here.
Grey-listing is a great way to prevent spam by temporarily "bouncing" a message. Most spammers do not use an email server to send their spam. They use an automated script that only attempts to send the message once. More information can be found here.
SRS - Sender Rewriting Scheme
SRS is an attempt to bring back email forwarding in conjunction with SPF.  It works by rewriting the envelope from when it "forwards" the message to another host. The envelope from, the server from, is different from the from in a message, the from you see on the E-Mail. E-Mail sent in this fashon will can allow other spam blocking programs to come into existence. SRS does pose a major problem as if you accept spam, you run the risk of getting your domain black listed. This is why you need to filter spam on your server before you accept email. More information can be found here.
For Consulting information and other questions, click here.