A dictionary email attack is when a perpetrator attempts to exploit your server to locate valid e-mail addresses. Once the hacker obtains a large group of e-mail addresses, they will send large volumes of unsolicited e-mail and spam to their illegally harvested e-mail collection. Dictionary attacks are extremely hard on your server's resources and can literally knock your server off-line. Aside from shutting down your server, your clients (who are being spammed) will be upset with your service which is a bad combination.
A dictionary attack is performed by using a program that auto generates e-mails and sends them to your server. Lets take Yahoo as an example. A program will start sending out e-mails to various Yahoo accounts starting with the first letter in the alphabet (A). An e-mail will be sent to aaron@yahoo.com, abby@yahoo.com, Aden@yahoo.com, and so forth. The e-mail phishing program will literally go through tens of thousands of different e-mail combinations in hopes to locate several working e-mail accounts.
These dictionary attacks are usually successful when the server they are attacking has a default catchall e-mail address set up. What happens is every time an invalid e-mail address is tried, the dedicated server routes the failed e-mail to the catchall address and bounces a reply back to the sender notifying them that the e-mail address is invalid. You can just imagine what kind of server load this induces when tens or hundreds of thousands of e-mails are being processed, routed, and bounced. This can result in the server literally going off line because too many resources were used.
Blacklisting IP addresses that frequently send dictionary attacks is almost a waste of time. Since it is very easy to change the header information in e-mails, one hacker can literally attack the same server with hundreds of different IP addresses. Within time, the file that retains the blacklisted IP addresses will become so large that it will take massive amounts of server storage space and a lot of resources to maintain.
One of the best ways to eliminate server load from dictionary attacks is to simply set your e-mail default address to fail. This is pretty easy to do and Cpanel as well as shell. Essentially, by setting your e-mail servers default address to fail, you are telling your server not to process and route invalid e-mails. Your server will still bounce an invalid e-mail reply to the sender, but it will not waste valuable resources forwarding in valid e-mails to your catchall address.
Dictionary attacks can certainly cause many heart aches and e-mail server problems already well known issue throughout the Internet community. Although there is no way to stop the dictionary attacks completely, you can eliminate the worries and stress associated with this ongoing problem while saving your server's resources.
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