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.

1.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.

2.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.
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
3.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.

To set up a smart host, follow these steps: 
In the MMC, click to select the SMTP virtual server, and 
then click Properties on the Action menu.
On the Delivery tab, click Advanced to open the Advanced 
Delivery dialog box.
In Smart host, type the name of the smart host server. You 
can type a string to represent a name, or you can type an 
IP address.
If you want the Microsoft SMTP Service to try to deliver 
remote messages directly before it forwards them to the 
smart host server, click to select the Attempt direct 
delivery before sending to smart host check box. The 
default is to send all remote messages to the smart host, 
not to try direct delivery.