Understanding DNS MX Records
MX (Mail Exchange) records are used to have email delivered to users on your domain. Every domain name that receives email must also have an MX record assigned to it in the domain name DNS records, and each MX record needs to resolve to either an IP address or a domain alias such as "mail.example.com"
When you send an email to someone, your email "typically" goes from your desktop email software (Such as MS Outlook), to your internet service providers outgoing mail server. Your internet service providers outgoing mail server then checks for an MX record of the domain in the email address and forwards the email to that location for delivery to your mailbox.
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 (Outgoing mail 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 for a domain name.
Example DNS MX records would be:
> "example.com - MX10" mail.example.com"
> "example.com - MX20" mail1.myisp.com"
> "example.com - MX30" mail2.myisp.com"
Your internet service providers outgoing mail server would first try mail.example.com, and if that wasn't reachable, it would try either mail1.myisp.com or mail2.myisp.com until it found a mail server that could accept email, as each MX record might be a totally different server.