MX Records

The MX record in a zone file delegates what server will be used to accept incoming email. Mail servers all over the world look up what the MX record is for a domain, and then send it to that server.

Let's lookup the MX record for sagonet.com:

kyle@kyle-laptop:~$ host -t mx sagonet.com
sagonet.com mail is handled by 0 mx.sagonet.com.

So mx.sagonet.com is the place to send mail for sagonet.com.

Here is what that line looks like in the zone file:

sagonet.com.            300     IN      MX      0 mx.sagonet.com.

The 0 right before the mx.sagonet.com represents a priority. You can setup mail servers with one as a backup if you want, like this:

sagonet.com.            300     IN      MX      0 mx.sagonet.com.
sagonet.com.            300     IN      MX      10 mx-backup.sagonet.com.

Any mail server will try the one with lowest priority first, and then try the next one if it can't connect.