Features
Greylisting
Greylisting is a simple method of defending electronic mail users against email spam. In short, an email transfer agent which uses greylisting will "temporarily reject" any email from a sender it does not recognize. If the email is legitimate, the originating server will try again to send it later, at which time the destination will accept it. If the email is from a spammer, it will probably not be retried, however, even spam sources which re-transmit later will be more likely to be listed in RBL's(Realtime Blackhole Lists) and distributed signature systems such as Vipul's Razor.
Greylisting requires little configuration and modest resources. It is designed as a complement to existing defenses against spam, and not as a replacement.
How it Works:
Typically, a server that uses greylisting will record the following three pieces of information (known as a "triplet") for each incoming email message:
- The IP address of the connecting host.
- The envelope sender address.
- The envelope recipient address.
This is checked against the email server's internal database. If this triplet has not been seen before (within some configurable period), the email is greylisted for a short time (also configurable), and it is refused with a temporary rejection. The assumption is that since temporary failures are built into the RFC specifications for email delivery, a legitimate server will attempt to connect again later on to deliver the email.
Greylisting is effective because many mass email tools used by spammers will not bother to retry a failed delivery, so the spam is never delivered. When a spammer does retry a delivery after the waiting period has expired, however, it will likely be after a number of automated honeypots have detected the spam source and listed both the source and the particular message in their databases. Thus, these subsequent attempts are more likely to be detected as spam by other mechanisms than they were at first. (en.wikipedia.org)
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