A smart host is a mail server designed to act as an intermediate stop along the way to mail delivery at a final destination. With a smart host, instead of sending mail directly from the sender's mail server to the recipient's, the mail is delivered to the intermediate server and it takes care of delivery. There are a number of advantages to this server design, and it is widely used all over the world in a variety of settings.
Handling mail can be a heavy load for a server. Mail servers have to process incoming and outgoing communication along with errors. Many are equipped to intercept spam and malicious email and this can eat up a lot of processing time. Maintaining a smart host allows people to keep a single robust mail server, rather than trying to maintain several smaller mail servers, a concern with some big companies. It can also reduce vulnerability to attack by limiting weak points in a company's computer system.
The smart host can also take over when a primary mail server goes down. Ordinarily, people trying to send or receive mail through a non-functioning server will get errors. With a smart host, the intermediate server can take over, handling mail requests until the primary server is back up. This reduces problems like confusing error messages, as well as eliminating user frustration related to problems with sending and receiving email, a potentially significant issue in an environment where people expect functioning email at all times.
The smart host can also offer authentication. Some email providers and Internet service providers, in an attempt to cut down on malicious email, have set up barriers designed to intercept problematic email. Using a smart host creates an opportunity for authentication, making sure the primary mail server is not tied up. In some cases, servers won't accept incoming mail sent directly from a sender, and expect to see it delivered by a known smart host.
People have many designs to choose from when it comes to a mail server. This option can be discussed with information technology staff to see if it is appropriate for a given mail handling need. If it is, they can set up the server and configure email programs to use it. Alternatives may be more suitable for some applications, and technical staff with experience and skills will know which choice would be the best for a situation.