Information about Mx Record
An MX record or Mail exchanger record is a type of resource record in the Domain Name System (DNS) specifying how Internet e-mail should be routed. MX records point to the servers that should receive an e-mail, and their priority relative to each other.
When an e-mail message is sent through the Internet, the sending mail transfer agent makes a DNS query requesting the MX record for the recipient's domain name, which is the portion of the e-mail address following the "@". This query returns a list of host names of mail exchange servers accepting incoming mail for that domain, together with a preference number. The sending agent then attempts to establish an SMTP connection to one of these servers, starting with the one with the smallest preference number, delivering the message to the first server with which a connection can be made. If no MX records were present, a second request is made for the A record of the domain instead.
The MX mechanism provides the ability to run multiple mail servers for a single domain and the order in which they should be tried, increasing the likelihood that mail may be delivered and providing the ability to distribute the processing of incoming mail across multiple physical servers. This ability to run multiple mail servers easily is proving very valuable for high-availability clusters of inexpensive mail gateways that can then process hundreds of messages per second in aggregate to quarantine or remove spam and/or viruses. However, not all versions of all mail transfer agents pay attention to lower priority MX records — in other words, if the highest-priority MX server fails, the MTA doesn't address the backup server.
The MX mechanism does not grant the ability to provide mail service on alternative ports, nor does it provide the ability to distribute mail delivery across a set of equal-priority mail servers by assigning a weighting value to each one. As of 2004, some mail transfer agents support the use of SRV records for publishing the IP addresses, ports, priority, and weights of mail servers.
One technique used to distribute the load of incoming mail over an array of servers is to return the same preference number for each server in the set. The available mail servers are then returned in a sort of "round robin" order, and since the priority values of all returned MXs are equal, the remote client will send its message using the first in the list. Upon the next request, the MXs will be returned in a "shuffled" order.
A favorite technique of spammers is to connect to the lowest priority MXs for a domain (those with the largest numerical value) in an attempt to avoid any anti-spam filters that may be running on the primary (highest priority) MX.
Overview
An MX record must contain a host name defined by an A record. CNAME aliases are not allowed to be used as MX record host names.When an e-mail message is sent through the Internet, the sending mail transfer agent makes a DNS query requesting the MX record for the recipient's domain name, which is the portion of the e-mail address following the "@". This query returns a list of host names of mail exchange servers accepting incoming mail for that domain, together with a preference number. The sending agent then attempts to establish an SMTP connection to one of these servers, starting with the one with the smallest preference number, delivering the message to the first server with which a connection can be made. If no MX records were present, a second request is made for the A record of the domain instead.
The MX mechanism provides the ability to run multiple mail servers for a single domain and the order in which they should be tried, increasing the likelihood that mail may be delivered and providing the ability to distribute the processing of incoming mail across multiple physical servers. This ability to run multiple mail servers easily is proving very valuable for high-availability clusters of inexpensive mail gateways that can then process hundreds of messages per second in aggregate to quarantine or remove spam and/or viruses. However, not all versions of all mail transfer agents pay attention to lower priority MX records — in other words, if the highest-priority MX server fails, the MTA doesn't address the backup server.
The MX mechanism does not grant the ability to provide mail service on alternative ports, nor does it provide the ability to distribute mail delivery across a set of equal-priority mail servers by assigning a weighting value to each one. As of 2004, some mail transfer agents support the use of SRV records for publishing the IP addresses, ports, priority, and weights of mail servers.
MX priority
The relative priority of an MX server is determined by the preference number present in the DNS MX record. When a remote client (typically another mail server) does an MX lookup for the domain name, it gets a list of servers and their preference numbers. The MX record with the smallest preference number has the highest priority and is the first server to be tried. The remote client will go up the list of servers until it successfully delivers the message or gets permanently rejected due to an unreachable server or if the mail account does not exist on that server. If there is more than one entry with the same preference number, all of those must be tried before moving on to lower-priority entries.One technique used to distribute the load of incoming mail over an array of servers is to return the same preference number for each server in the set. The available mail servers are then returned in a sort of "round robin" order, and since the priority values of all returned MXs are equal, the remote client will send its message using the first in the list. Upon the next request, the MXs will be returned in a "shuffled" order.
A favorite technique of spammers is to connect to the lowest priority MXs for a domain (those with the largest numerical value) in an attempt to avoid any anti-spam filters that may be running on the primary (highest priority) MX.
See also
- LOC record
- SRV record
- Sender Policy Framework
- Email hub: MX based load-balancing and simple failover.
References
- RFC 974 (1986), Mail Routing and the Domain System (obsolete)
- RFC 2821 (2001), Simple Mail Transfer Protocol
On the Internet, the Domain Name System (DNS) associates various sorts of information with so-called domain names; most importantly, it serves as the "phone book" for the Internet by translating human-readable computer hostnames, e.g. en.wikipedia.
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Internet is a worldwide, publicly accessible series of interconnected computer networks that transmit data by packet switching using the standard Internet Protocol (IP). It is a "network of networks" that consists of millions of smaller domestic, academic, business, and government
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E-mail (short for electronic mail; often also abbreviated as e-mail, email or simply mail) is a store and forward method of composing, sending, storing, and receiving messages over electronic communication systems.
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A mail transfer agent or MTA (also called a mail transport agent, message transfer agent, mail server, SMTPD (short for SMTP daemon), or a mail exchanger
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domain name has multiple related meanings:
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- A name that identifies a computer or computers on the internet. These names appear as a component of a Web site's URL, e.g. wikipedia.org. This type of domain name is also called a hostname.
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e-mail address identifies a location to which e-mail messages can be delivered. The term "e-mail address" is also used as the formal pre-registered authoritative electronic mailing delivery site for an individual (example: an attorney's e-mail address registered for delivery of
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A hostname (occasionally also, a sitename) is the unique name by which a network-attached device (which could consist of a computer, file server, network storage device, fax machine, copier, cable modem, etc.) is known on a network.
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A mail transfer agent or MTA (also called a mail transport agent, message transfer agent, mail server, SMTPD (short for SMTP daemon), or a mail exchanger
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Simple Mail Transfer Protocol (SMTP) is the de facto standard for e-mail transmissions across the Internet. Formally SMTP is defined in RFC 821 (STD 10) as amended by RFC 1123 (STD 3) chapter 5. The protocol used today is also known as ESMTP and defined in RFC 2821.
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E-mail spam, also known as bulk e-mail or junk e-mail is a subset of spam that involves sending nearly identical messages to numerous recipients by e-mail. A common synonym for spam is unsolicited bulk e-mail (UBE).
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port is a special number present in the header of a data packet. Ports are typically used to map data to a particular process running on a computer.
Ports can be readily explained with an analogy: think of IP addresses as the street address of an apartment building, and the
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Ports can be readily explained with an analogy: think of IP addresses as the street address of an apartment building, and the
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A mail transfer agent or MTA (also called a mail transport agent, message transfer agent, mail server, SMTPD (short for SMTP daemon), or a mail exchanger
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An SRV record or Service record is a category of data in the Internet Domain Name System specifying information on available services. It is defined in RFC 2782. Newer internet protocols such as SIP and XMPP often require SRV support from clients.
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An IP address (Internet Protocol address) is a unique address that certain electronic devices use in order to identify and communicate with each other on a computer network utilizing the Internet Protocol standard (IP)—in simpler terms, a computer address.
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In the Domain Name System, a LOC record (RFC 1876) is a means for expressing geographic location information for a domain name.
It contains WGS84 Latitude, Longitude and Altitude information together with host/subnet physical size and location accuracy.
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It contains WGS84 Latitude, Longitude and Altitude information together with host/subnet physical size and location accuracy.
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An SRV record or Service record is a category of data in the Internet Domain Name System specifying information on available services. It is defined in RFC 2782. Newer internet protocols such as SIP and XMPP often require SRV support from clients.
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In computing, Sender Policy Framework (SPF) is an extension to the Simple Mail Transfer Protocol (SMTP). SPF allows software to identify and reject forged addresses in the SMTP MAIL FROM (Return-Path), a typical nuisance in e-mail spam.
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The term Mail Hub is used to denote an MTA or system of MTAs used to route email but not act as a mail server (having no end-user email store) since there is no MUA access.
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