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• Blocklist: A database of ip addresses or web urls suspected of being involved in spam or other abuse; generally a blocklist is not used by end-users, but is instead queried by mail hosts in order to reject or mark probable spam messages upon delivery.
• Probe messages are evidence of a dictionary attack in which the spammer sends probe messages to a large number of e-mail addresses and removes those that are rejected by the mail system.
• A mail host is supposed to reject messages immeidately if they cannot be delivered, or if the mail host does not wish to deliver them for some reason.
• The best practice is for the mail system administrator to program his mxs to reject the mail at the time of the delivery attempt, rather than to pass it on to an mda to be bounced.
• When the graylisting mx rejects a message from a spammer, the spammer generally won't attempt redelivery (because he has plenty of other addresses to try, and can't be bothered with following up on rejections).
• Graylisting is a technique used by a mail exchanger host to defend against spam deliveries by temporarily rejecting all mail from unrecognized sources.
• Graylisting requires a bit of extra work by the mx (particularly for large mailing services that must coordinate among many mx hosts), but possibly not as much cpu time as would be required to process and filter the spam if it were not rejected.
• For example, if you define cialis as a spam word, then every e-mail that has the word cialis in it will be rejected or placed in your junk mail folder.
• Probe messages are evidence of a dictionary attack (or "directory harvest attack" or "mx probe") in which the spammer sends probe messages to a large number of e-mail addresses (some of which may be guesses) and prunes out those that are rejected by the mx.
• If a sender's ip appears in a blocklist, the mail host can reject delivery of the message so that it will not show up in the recipient's inbox.
• Blacklists are not a terribly effective form of spam filter as they usually do not block or reject e-mail as much as merely route it to some destination of the user's choice (like the trash can).
• If the receiving host has good reason to believe that an offered message is spam, it can reject the message which will spare the recipient from having to download it and deal with it.
• A good spam filter software program will allow you to set up features regarding desired email, including which messages to accept and reject (white and black lists), as well as which to organize into folders.

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