I read a tip just recently that advocated the use of fake MX records as a spam deterrent.  The solution was apparently devised after struggling with the server load that was being generated by spam emails.

As we all know, spam makes up as much as 90% of global email traffic, so it is not unusual for spam load to be a serious issue for email server performance.  The natural instinct is to prevent that load from being applied to the server in the first place.  Fake MX records are not the best way to do this.

MX records are the DNS records that tell email servers where to send email that is addressed to a particular domain.  For example, if I send an email to john@company.com my email server will look up the MX record for company.com, determine the associated IP address, and transmit the message over SMTP to that IP address.

To maintain redundancy most organizations will use multiple MX records that point to multiple email servers, so that if one is unavailable the others can still receive incoming email.  MX records are given a priority, an arbitrary number that is only relative to the priority of other MX records for that domain.  The lower the number, the higher the priority.

So for the same example as above, my email server looks up the MX record for company.com and gets the following response.

company.com MX preference = 10, mail exchanger =
 maila.company.com

company.com MX preference = 20, mail exchanger =
 mailb.company.com

It knows then to send to maila.company.com first, and then try mailb.company.com if the first try is not successful.

The idea of fake MX records is to create multiple MX records (usually at least 3) of varying priority, and have the highest and lowest priority MX records be pointing to non-existent servers.  The theory is that spammer’s botnets will only try to send to the highest or lowest priority MX, and then when they get no response will give up and move on to the next victim.  Some email administrators use as many as 10 MX records with only one real one among them.

The theory has some merit.  Spammers want to send out as much email as possible so usually won’t waste time and resources by having their bots try multiple MX records for a targeted domain.  However the technique impacts legitimate senders as well. Continue reading Fake MX Records More Harm Than Good