These are special resource records for mail routing. An MX (Mail eXchanger) record specifies a domain name and an associated computer, the “mail exchange”, which is able to deliver e-mail to an e-mail address with this domain name.
A Sample MX record that you can register with 1&1 looks like this:
Domain Name MX1: mail.domain.com Priority: 10 IP Address 192.168.1.1
Domain Name MX2: backup.domain.com Priority: 20 IP Address 192.168.1.2
Domain Name MX1: mail.otherdomain.com Priority: 30 IP Address 18.104.22.168
To differentiate them, each MX record has a priority (lower the number, higher the priority). The MX record with the highest priority is the actual target computer where mail boxes are located. The other MX records designate the backup computers in case the actual target computer is not accissible. They store the e-mails temporarily, and periodically attempt to redirect e-mails to the actual target computer.
An IP address is used for a subdomains like mail.domain.com