By default, email sent with System.Web.Mail is formatted as plain text. To format as Html, set the MailMessage.BodyFormat property to MailFormat.Html.
 
[ C# ]

MailMessage mail = new MailMessage();
mail.To = "me@mycompany.com";
mail.From = "you@yourcompany.com";
mail.Subject = "this is a test email.";
mail.BodyFormat = MailFormat.Html;
mail.Body = "this is my test email body.<br><b>this part is in bold</b>";
SmtpMail.SmtpServer = "localhost";  //your real server goes here
SmtpMail.Send( mail );

[ VB.NET ]

Dim mail As New MailMessage()
mail.To = "me@mycompany.com"
mail.From = "you@yourcompany.com"
mail.Subject = "this is a test email."
mail.BodyFormat = MailFormat.Html
mail.Body = "this is my test email body.<br><b>this part is in bold</b>"
SmtpMail.SmtpServer = "localhost" 'your real server goes here
SmtpMail.Send(mail)