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ASP.NET app to send an mail with a hyperlink
MailMessage message = new MailMessage();
                message.From = new MailAddress("hkar@gmail.com");

                message.Subject = "Subject";
                message.Body = "Please login";
                SmtpClient smtp = new SmtpClient();

                message.To.Add("karaman@gmail.com");
                smtp.Send(message);

message.Body = "Please <a href="http://www.example.com/login.aspx">login</a>";

message.IsBodyHTML = true;

<a href="http://YourWebsite.Com">Login</a>

System.Text.StringBuildersb = new System.Text.StringBuilder();
 System.Web.Mail.MailMessage mail = new System.Mail.Web.MailMessage();
 mail.To = "recipient@address";
 mail.From = "sender";
 mail.Subject = "Test";
 mail.BodyFormat = System.Web.Mail.MailFormat.Html;
 sb.Append("<html><head><title>Test</title><body>"); //HTML content which you want to send
 mail.Body = sb.ToString();
 System.Web.Mail.SmtpMail.SmtpServer = "localhost"; //Your Smtp Server
 System.Web.Mail.SmtpMail.Send(mail);

message.Body = string.Format("Click <a href='{0}'>here</a> to login", loginUrl);