Other Antispam Approaches
SpamAssassin combines message format validation, content-filtering, and the ability to consult network-based blacklists . Filtering systems require little user intervention and introduce little delay into the process of sending and receiving email. There are other approaches to preventing spam, each of which comes with its own advantages and disadvantages (and many of which can be used in addition to, as well as in place of, SpamAssassin).
In a challenge/response system, the system holds all messages from unknown senders and sends them a reply message with a unique code or set of instructions (the challenge ). The senders must reply to the challenge in some fashion that verifies their email addresses and ( generally speaking) proves that they are human beings, rather than an automated bulk mail program (the response ). After a successful response, the system allows messages from the sender to be accepted, rather than holding them.
In greylisting systems, the mail server initially returns a temporary SMTP (Simple Mail Transfer Protocol) failure code to messages from new senders or sending systems. If the sending system attempts to resend the message after a reasonable time period, the mail server accepts the message and subsequent messages from the sending host. Because spammers are likely either to treat the temporary failure as a permanent failure, or to attempt to deliver messages continually during the greylisting time period, their messages are not received.
In time-limited address systems, users generate unique variations of their email address to include in different web forms, email messages, newsgroup postings, etc. Addresses may be valid only for a limited time or may be valid until revoked by the user. In these systems, if a user receives spam at one of his addresses, he can usually identify the company that spammed him (or sold his address to a spammer), and he can quickly invalidate the address to prevent further spam.
In micropayment systems, senders must pay a small fee for each message they send, making large-scale spam runs costly. In some of these systems, the micropayment is refunded when the recipient determines that the message is in fact non-spam. (SpamAssassin 3.0 supports a variation of micropayments in the form of Hashcash, in which the payment is made in processing time rather than money.)