The MX resource record or Mail Exchange resource record (MX-RR) of a Domain is apart from (under others) A resource record (the IP address) an entry (resource record) in the Domain Name System, which refers exclusively to the service E-Mail (smtp).

The MX record makes it possible to operate under a Domain several Mailserver. In addition it gives a prioritization, in which order these are to contact the Mailserver of a certain Domain to other Mailservern. If one precipitates this server, that increases the probability that a Mail can be set nevertheless to the Finally this procedure makes possible to take Mailserver for maintenance work from the net to individual (physically separated) without endangering the receipt from Mails to.

With the dispatch of a E-Mail the exit Mailserver queries first the MX-RR of the Domain (the part of the E-Mail after address "„@ "“), which are assigned the receiver address. As a rule mindest a MX-RRs exists to each Domain (also Sub Domains). The output server develops then a smtp connection with the first listed Mailserver of the Eingangsdomain. If no MX-RR is present, the Mailserver tries to determine A resource record (the IP address) the Domain. If the DNS returns a IP address on this request, the Mailserver tries to develop to this IP address a smtp connection. Even if that does not succeed, he announces an error (host unknown).

If a MX-RR is present, he sends the E-Mail to the Mailserver, which is noted there. If several MX-RRs are present, the E-Mail is sent to the Mailserver with the lowest priority value. If this is not attainable, the Mailserver tries to reach the next Mailserver in the MX list etc. by it can the loss of a Mailservers be relatively simply compensated.

Example:

If a Mail at Vorname.Nachname@wikipedia.de is to be dispatched, the Mailserver of the sender examines first the MX record of wikipedia.de. He gets the following Mailserver mentioned and tries to contact it in this order:

       C:\ >nslookup > set type=MX > wikipedia.de wikipedia.de MX preference = 10, mail more exchanger = mail.wikipedia.de wikipedia.de MX preference = 20, mail more exchanger = smtprelay.regiomedia.de wikipedia.de MX preference = 30, mail more exchanger = mx30.regiomedia.de 

A special form of MX-RR are Reverse MX-RRs. Here it concerns the Domain Name of system entries of the dispatching Mailserver. These entries are used e.g. with the transmitters Policy Framework Technology.

One with Spamern method liked is to develop a connection to that Mailserver which is indicated as the smallest priority in MX list (that with the highest numeric value). Thereby Spam filters are to be gone around, which run on the Mailserver with the highest priority.

Priority and Preference: The Mailserver with the highest priority is that with the lowest "“Preference"” - number.

Related links

• RFC 974 - Mail Routing and the Domain system

Articles in category "MX resource record"

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