What Tests are most effective against Dictionary Attacks?
What is a Dictionary Attack? A Dictionary Attack is a spamming technique where spammers submit thousands or millions of email messages with random addresses. The object of the attack is to find which addresses are valid either by a DSN or there may be hidden code in the message that reports back to the spammer when the email has been opened, which lets the spammer know which email addresses are valid. These are then added to the spammer’s list, which is then resold to spammers world wide. Greylisting is a very good defense against Dictionary attacks. Using the user restriction test with the user delete recipient option is effective because the unknown user is deleted from the message without sending a notice to the sender. For DSN (Delivery Status Notice) attacks use the discard_dsn option. Using the rebuild_access option will help performance by adding blacklisted domains and ip addresses to your access file and discarding the connection or domain as it comes in.
