It can be because of various reasons. You need to look at them one by one.
Is the port open? Firewall permissions in place?
Further make sure you have configured SMTP configuration in Web.Config:
<network host="somesmtpserver" port="25" userName="name" password="pass" defaultCredentials="true" />
If needed, have a look at this Microsoft Video tutorial:
Use ASP.NET to send Email from Website
If needed, there are lots of article on this very site on how to send emails