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Information on Dictionary Attacks
a) What is a Dictionary Attack?
Directory Harvest Attacks (DHAs) are carried out against mail servers to acquire a list of users on your server that will accept mail; that list is sold to spammers. Directory Harvest Attacks (DHAs), sometimes called Dictionary Attacks, are carried out by spammers against mail servers. Various combinations of username are tried - email@example.com, firstname.lastname@example.org; if the server responds differently for a valid or invalid user, a list of valid users can be built up.
b) How can I prevent Dictionary Attacks?
Various mail servers now have inbuilt protection against DHAs, and filtering gateways are available that can help.
Please click on the link http://spamlinks.net/prevent-secure-dha.htm to have access to relevant materials on mail servers for the following applications: