BACKGROUND: MX (Mail Exchange) records are used to have mail delivered to users on your domain. It MUST have an MX record (not just an A record), primarily because people typically use an E-mail address with your domain name (“[email protected]”), not a subdomain (“[email protected]”).
When you send mail to someone, your mail typically goes from your E-mail client to an SMTP server. The SMTP server then checks for the MX record of the domain in the E-mail address. For example, with “[email protected]”, it would look for the MX record for example.com. If a user did have an E-mail address “[email protected]”, the SMTP server would look for the MX record of “mail.example.com”. The MX record is a domain name, so the SMTP server then gets the A record for that domain name, and connects to the mail server.
Each MX record has 2 pieces of information associated with it. The first is a number (“Preference” number), the second is the domain name of the mail server. If there are multiple MX records, the SMTP server will pick one based on the preference level (starting with the lowest preference number, working its way up). It’s O.K. to have more than one MX record with the same preference.
An example would be “example.com MX 10 mail.example.com”, “example.com MX 50 mail1.myisp.com”, and “example.com MX 50 mail2.myisp.com”. An SMTP server would first try mail.example.com, and if that wasn’t reachable, it would try either mail1.myisp.com or mail2.myisp.com (normally, it should pick one of the two randomly, unless it has a better reason to pick one over the other, since the preferences are the same).