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 126.96.36.199