Information Security

Method 1: The non-primary MX attack

A significant number of spam emails specifically target non-primary MX hosts for domains, for the simple reason that backup MX servers will usually accept and relay all of the spam to the primary MX host without checking it, which reduces the load on the spammer's system, requires little or no additional processing for mails that are rejected, and usually results in faster delivery transactions because the receiving system has to do less work (in the short term while the attack is occuring).

Greylisting handles this attack very well, since the whole point of the attack is to minimize bounces and delivery delays.

Method 2: The spam troll/Dictionary attack

Many spammers are now resorting to "trolling", that is, sending spams to common usernames (tom@, harry@) at domains (also known as a dictionary attack), or sending to generated usernames made from real names harvested from other sources. They usually seem to be operating from a dictionary of common user names, but the "generated" usernames tactic may be getting more common.

The spammers probably use this method in order to reach people who have either taken steps to try to keep their email address from being harvestable from the web, or who are fairly novice users that may not have the resources or inclination to create their own web pages. Probably the latter case, since novice users are probably more likely to purchase something that has been advertised through spam.

This type of attack is very often combined with the non-primary MX attack, since most of these emails will result in bounces on domains that don't have a fairly large user population. Consequently, the spammers target the backup MX hosts. That way, they don't have to handle all the bounces and failures that these messages generate.

Greylisting handles these very well, since they almost always come from random short-lived dynamic IP addresses. And because most of these emails will ultimately generate bounces, it is costly for spammers to attempt redelivery of this type of attack. Also, since this attack is so distinctive (A high number of bounces generated in a short period of time from a particular IP address or set of addresses), it should be very easy to recognize and add to other blacklisting methods if given enough time to do so, which Greylisting provides.

Method 3: The organized distributed attack

Many