MailMessage mail = new MailMessage(); 
mail.To = "To Address"; 
mail.From = "From Address"; 
mail.Subject = "Test."; 
mail.BodyFormat = MailFormat.Html; 
mail.Body = "Test Message"; 
SmtpMail.SmtpServer = "localhost"; //your real server goes here 
SmtpMail.Send( mail );

Install the SMTP virtual server that is part of IIS. 
·	You will need your Windows installation CD. 
·	Use Add or Remove Programs in the Windows Control Panel to launch Add/Remove Windows Components. 
·	Select Internet Information Services (IIS) and then click Details. 
·	Check SMTP Service and then click OK. 
·	The installation process will prompt you for your Windows CD and will install the SMTP virtual server. 

After installing the SMTP server (also reboot), configure it by following these steps: 
1.	In the Control Panel, choose Administrative Tools, and then choose Internet Information Services. 
2.	Open the node for your computer, right-click the Default SMTP Virtual Server node and choose Properties. 
3.	In the Access tab, click Connection. 
4.	Select Only the list below, click Add and add the IP 127.0.0.1. This restricts connections to just the local computer, i.e., localhost. Close the Connection dialog box. 
5.	Click Relay and repeat Step 5 to allow only localhost to relay through this server. 
In your firewall or router (or both), close incoming port 25. This is an important security measure that will prevent spammers from finding your SMTP server and using it to relay spam.

This is a network related error. Your application cannot connect to the mail server specified.

1. Make sure that the following are true about SmtpMail.SmtpServer:
2. Is a valid SMTP Server 
3. Make sure that the server System.Web.Mail is running or can connect to the mail server. Some times firewalls or proxy servers can get in the way. 
4. Try specifying the value by IP address. Poor DNS resolution can sometimes hinder name lookups. 
5. Make sure that the mail server is running at port 25. 
6. If you did not specify a SmtpMail.SmtpServer property, or if SmtpMail.SmtpServer points to "localhost" (or the equivalent), be sure the SMTP Service is running on port 25. 
7. For testing purposes change the MailMessage.From and MailMessage.To properties to an address that exists on SmtpMail.SmtpServer. Some times this exception is thrown, when it really is a relay issue.