Typically this exception has to deal with the following line of code (or I should say ABSENCE of the following line of code):

SmtpMail.SmtpServer = "mail.your-domain.com"

By default, if SmtpMail.SmtpServer is not set, System.Web.Mail is supposed to use localhost as the default property. However, for some reason this doesn't appear work. If you are using the local SMTP Service to send emails, you may try the following values:

"127.0.0.1"
"localhost"
"the_machine_name_here"
"the_real_dns_name_here"
"the_machine_IP_Address_here"

That should fix it.