MX record

The MX resource record or Mail Exchange resource record ( MX RR ) of a domain is an entry (Resource Record) in the Domain Name System, which refers exclusively to the e-mail service (SMTP).

An MX record indicates under which Fully Qualified Domain Name (FQDN ) of the mail server to a domain or subdomain is achievable. It is customary to define a domain for multiple MX records with different priorities, so that when a mail server fails, another can receive the e -mails. This increases the likelihood that a mail can still be delivered to the recipient domain. Finally, this method, individual ( physically separate ) mail server for maintenance to take from the network without jeopardizing the receipt of mails possible.

When sending an e- mail to the outgoing mail server first queries the MX RR of the domain (the part of the email address after the "@"), which is associated with the recipient's address. As a rule, one or more MX RR exists for each domain (including sub-domains ). The origin server then establishes an SMTP connection to the mail server first listed the input domain. If no MX RR is present, the mail server will try the A resource record to determine (the IP address) of the domain. The use of the A resource records will, however, first mentioned in RFC 2821 from 2001. Therefore, it is questionable in practice, if appropriate mail implementation to RFC 974, which were not, or not significantly modified or expanded after 2001 based treat this at all correct. If the DNS returns the IP address to this request, the mail server will try to establish an SMTP connection to that IP address. Also if unsuccessful, it reports an error (host unknown).

If an MX RR is present, it sends the email to the mail server that is noted there. If multiple MX RRs are available, the e -mail to the mail server with the lowest value ( " preference" ) will be sent. Therefore, this value is often referred to as distance. If this is not available, the mail server the next mail server in the MX list is trying to achieve, etc. Thus, the failure of a mail server can be relatively easily compensated.

Construction

Example

Manual review of MX records

Normally working mail server corresponding DNS queries for MX records when sending e -mails.

MX records can be retrieved manually with DNS query tools. However, since they usually default query only A records, the request type must be specified explicitly in this case.

Windows / Unix ( including Mac OS X)

Under Microsoft Windows, Unix and Mac OS X can use the supplied command line tool nslookup an MX record to be queried. The setting of the type done by set type = MX.

On Unix and Linux derivatives there are different ways to query MX records. The dig program is the complete response from the DNS server returns:

A quick search is also possible with the program host:

A special form of MX RRs are reverse MX RRs. This is the domain name system records the outgoing mail server. These entries are for example used in the Sender Policy Framework technology.

Priority and Preference

A popular method for spammers is to connect to that mail server specified in the MX list with the lowest priority ( the one with the highest numerical value ). This is intended to spam filters are bypassed, running on the mail server with the highest priority.

The mail server with the highest priority is the one with the lowest preference value.

541103
de