A Mail Exchange Record (MX Record) is a DNS record that holds information about which mail server the domain uses to receive mail. MX Records only indicate which mail servers should receive mail for a domain but not which mail servers should be used when sending mail from a specific domain. Often you can use multiple different servers to send your mail but only one address for receiving. Normally there is more than one mail server address that is used with a domain.
When editing MX Records with 1&1, two pieces of information are necessary: the mail server address and the mail server priority.
Since usually more than one mail server address is used with a domain, the priority specifies which mail server should be used first. If the primary mail server would become inaccessible or malfunction, then the second mail server with a lower priority would then be used.
Mail server priorities are usually listed in multiples of 10 with the lowest number taking priority (much like golf). View the sample MX Records below.
Using the example above, the mail server at mx00.domain.com would be used as the primary mail server as its priority is the highest (even though the number is lowest). As long as you are using the 1&1 name servers, you can edit/change a domain's mail servers (MX Record) in your 1&1 Control Panel. MX Records must be entered as a domain or subdomain, not an IP address. If your mail servers only use an IP address, there is a work-around of entering as an IP Address for a Domain's MX Record.