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A smart host is a type of mail relay server which allows an SMTP server to route e-mail to an intermediate mail server rather than directly to the recipient’s server. Often this smart host requires authentication from the sender to verify that the sender has privileges to have mail forwarded through the smart host. This is an important distinction from an open relay that will forward mail from the sender without authentication. Common authentication techniques include SMTP-AUTH and POP before SMTP.
Use for backup mail (secondary MX) services
When configured to be a backup mail server (not the primary MX record) a smart host configuration will accept mail on behalf of the primary mail server if it were to go offline. When the primary mail server comes back online, mail is subsequently delivered via the smart host.
Use in spam control efforts
Some ISPs, in an effort to reduce e-mail spam originating at their customer’s IP addresses, will not allow their customers to communicate directly with the recipient’s mail server via the default SMTP port number 25. In this case the customer has no choice but to use the smart host provided by the ISP.
ISP's involved in this practice include:
A growing number of systems also verify the sending system against known lists of cable modem and DSL networks and will not accept SMTP connections from these systems to reduce the amount of incoming spam. Field tests have shown this can have a sizable impact on the number of spam messages one receives and it is expected to become more and more common.
Use in centralizing email services
When a host runs its own local mail server, a smart host is often used to transmit all mail to other systems through a central mail server. This is used to ease the management of a single mail server with aliases, security, and Internet access rather than maintaining numerous local mail servers.
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