The MX Resource Record or Mail Exchange Resource Record (MX-RR) of a domain is (among others) besides the A Resource Record (the IP address) an entry (Resource Record) in the Domain Name System that solely relates to the eMail (SMTP) service. The MX record provides the possibility of running several mail servers under one domain. Moreover it specifies the priority in which order other mail servers are supposed to contact the mail servers of a certain domain. In case one of these servers fails the likeliness of a successful delivery is even increased. Eventually this method makes it possible to isolate single (physically separated) mail servers from the net for maintenance without endangering the retrieval of eMails.
When sending an eMail the outbox mail server requests the domain's MX-RR (the part following the "@") to whom the recipients address is assigned. By default at least one MX-RR exists for each domain (sub domains included). The outbox server then establishes an SMTP connection with the first listed mail server of the recipient domain. If there's no MX-RR the mail server is trying to get the A Resource Record (the IP address) of the domain. If the DNS returns an IP address for this request the mail server tries to establish an SMTP connection with this IP address. In case this operation fails there will be an error message (host unknown).
If a MX-RR is existing it sends the eMail to the registered mail server. In case there are several MX-RRs the eMail is being sent to the mail server with the least priority value. If it doesn't respond the mail server tries to reach the next mail server from the MX list. By doing so the failure of a mail server can be compensated quite easily.
In the customer center you have the option to enrol your mail servers for your domain in the DNS zone. To this please enter the full hostname of your mailserver (mail.domain.de / mail2.domain.de) in the field "MX-Servername". This hostname must be existent otherwise loss of your eMails could occur.
The main eMail server usually gets the priority MX 10. The BackUp mail server normally gets priority MX 20.
Please note that the TTL has to amount to at least 60. Lesser values aren't accepted by the system.