In the previous post, we talked about MX records. When we looked at gmail.com’s MX records, there were numbers before the hostnames. Those numbers are preferences, also known as MX preference.

How’s that for a technically boring term? MX preference means something when the computer is determining which of the records to use as a destination for my email. Let’s look at those results for gmail.com again:

gmail.com mail is handled by 40 alt4.gmail-smtp-in.l.google.com.
gmail.com mail is handled by 20 alt2.gmail-smtp-in.l.google.com.
gmail.com mail is handled by 30 alt3.gmail-smtp-in.l.google.com.
gmail.com mail is handled by 10 alt1.gmail-smtp-in.l.google.com.
gmail.com mail is handled by 5 gmail-smtp-in.l.google.com.

The one with the lowest number has the highest preference. How’s that for a contradictory sentence? But it’s how it works. This means that if I’m sending email to gmail.com, then the first place it’s going to try is the system gmail-smtp-in.l.google.com. If this fails, then it goes to the next in the list by preference, that is, alt2.gmail-smtp-in.l.google.com.

The geniuses (Not Dr. Strange and Ironman!) who designed this knew that people really wanted to get their email. Well, not spam email, but they didn’t consider that. They knew that people really wanted email and it should work, so they made it so what should happen is that the system sending the email should try each in order of MX preference.

Not everyone does it, of course. The Internet is a cooperative and it depends on people doing the ‘right thing’ or an approximation close enough to keep things running. Sometimes you’ll send an email and see a bounce from the mailer-daemon (not a real demon), but if you try again in just a few minutes, it’ll go through.

Either the other domain didn’t have a server up to receive the email or the system didn’t check through the MX preference list like it should have, or there was a real demon in the system. Any one of these could have caused the problem.  Especially the demon, demons are trouble.

Fred the Demon

I call mine Fred.

Disclaimer: There are no real demons in the Internet. …I think.