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A smart host or smarthost is a server with a reliable internet connection and running robust Simple Mail Transfer Protocol (SMTP) and Domain Name System (DNS) software, via which third parties can send emails (typically by connecting to the smarthost and sending over a batch of one or more outbound emails), thus avoiding the need to maintain such servers themselves. The message transfer agent software on the smarthost then sends those emails to the email recipients' email servers. Smarthosts were originally open mail relays, but abuse by spammers meant that most providers switched to requiring authentication from the sender, to verify that the sender is authorised to have mail forwarded through that particular smarthost. (For example, an ISP might run a smarthost whose usage is authorised for the ISP's paying customers only.) Common authentication techniques include SMTP Authentication 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 internet service providers (ISPs), in an effort to reduce email 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 can get round this restriction by using a tunnel or VPN, or possibly by using an alternative TCP port number in SMTP, but otherwise the customer has no choice but to use the smart host provided by the ISP.
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.
- Moen, Rick (June 2005). "Mail Relaying/Authentication". Linux Gazette (115).
The term 'smarthost' harks back to the days before SMTP was universal and DNS reasonably reliable: The idea was that you assumed that your own host probably wasn't as well-connected and capable as, say, the UCBVAX machine at Berkeley. So, you would configure your local outbound mail processes to lob all outgoing mail over to UCBVAX via batched, dialed-up UUCP, or SMTP, or whatever, trusting to UCBVAX to perform necessary DNS lookups and redeliver the mail. UCBVAX was, in that sense, dubbed a "smarthost" for your site, in the sense that it's smarter about DNS and mail connections than yours is.
Not only UCBVAX but also pretty much every other mail site used to perform this service routinely for anyone and everyone, as a convenience for the Internet community. Then, along came spammers. Pretty soon, having a mail server available for redelivery of mail to and from everyone came to be seen as a public menace, like having an unfenced swimming pool in a neighbourhood full of children. The term for such a thing became 'open relay', and self-appointed spamcops started maintaining databases (blocklists) of IPs that had been tested as being willing to relay for random members of the public.
When we relay mail for you at my server, what we want is not an open relay, but relaying specifically for outbound mail coming from you. The question, then, is: By which of the several possible means is my MTA going to know that the mail being received is from you? ...
There's something called 'SMTP AUTH', a modification of standard SMTP specifically for situations like [these], where you provide some sort of client authentication, as part of an extended variant on SMTP negotiation, on mail you want to relay outbound through my mail host.