A smart host or smarthost is an
email server
Within the Internet email system, a message transfer agent (MTA), or mail transfer agent, or mail relay is software that transfers electronic mail messages from one computer to another using SMTP. The terms mail server, mail exchanger, and MX host ...
via which third parties can send emails and have them forwarded on to the email recipients' email servers.
Smarthosts were originally
open mail relay
An open mail relay is a Simple Mail Transfer Protocol (SMTP) server configured in such a way that it allows anyone on the Internet to send e-mail through it, not just mail destined to or originating from known users. This used to be the default con ...
s,
but most providers now requiring authentication from the sender, to verify that the sender is authorised
- for example, an
ISP
An Internet service provider (ISP) is an organization that provides services for accessing, using, or participating in the Internet. ISPs can be organized in various forms, such as commercial, community-owned, non-profit, or otherwise private ...
might run a smarthost for their paying customers only.
Use in spam control efforts
In an effort to reduce
email spam originating from their customer's
IP address
An Internet Protocol address (IP address) is a numerical label such as that is connected to a computer network that uses the Internet Protocol for communication.. Updated by . An IP address serves two main functions: network interface ident ...
es, some
internet service provider
An Internet service provider (ISP) is an organization that provides services for accessing, using, or participating in the Internet. ISPs can be organized in various forms, such as commercial, community-owned, non-profit, or otherwise privat ...
s (ISPs), will not allow their customers to communicate out directly with recipient mailservers via the default SMTP
port number
In computer networking, a port is a number assigned to uniquely identify a connection endpoint and to direct data to a specific service. At the software level, within an operating system, a port is a logical construct that identifies a specific ...
25. Instead, often they will set up a smarthost to which their customers can direct all their outward mail - or customers could alternatively use one of the commercial smarthost services.
Sometimes, even if outward port 25 is not blocked, an individual or organisation's normal external IP address has difficulty getting
SMTP
The Simple Mail Transfer Protocol (SMTP) is an Internet standard communication protocol for electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-level email clients ty ...
mail accepted. This could be because that IP was assigned in the past to someone who sent spam from it, or appears to be a dynamic address such as typically used for home connection. Whatever the reason for the "poor reputation" or "blacklisting", they can choose to redirect all their email out to an external smarthost for delivery.
Reducing complexity
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.
See also
*
Mail submission agent
A message submission agent (MSA), or mail submission agent, is a computer program or software agent that receives electronic mail messages from a mail user agent (MUA) and cooperates with a mail transfer agent (MTA) for delivery of the mail. It us ...
References
{{DEFAULTSORT:Smart Host
Email
Internet terminology