If you are trying to send email without authenticating, I am afraid that's impossible. If any users in your web site can send emails without password, that's horrible. It will allow user to send email from other person account. So considering security, sending email will required to provide email address and password
var fromAddress = ""; // Email Address here. This will be the sender.
string fromPassword = ""; // Password for above mentioned email address.
var toAddress = "";// Receiver email address here
string subject = "Hi";
string body = "Body Text here";
var smtp = new System.Net.Mail.SmtpClient();
{
smtp.Host = "smtp.gmail.com"; // this is for gmail.
smtp.Port = 587;
smtp.EnableSsl = true;
smtp.DeliveryMethod = System.Net.Mail.SmtpDeliveryMethod.Network;
smtp.Credentials = new NetworkCredential(fromAddress, fromPassword);
smtp.Timeout = 20000;
}
smtp.Send(fromAddress, toAddress, subject, body);
[Edited]
Sorry My mistake i didn't noticed that. They both used for same purpose. If you are using higher version (2.0 or later) of .Net framework, then use System.Net.Mail. If you use System.Web.Mail it only shows a warning saying this is deprecated. But that will work.
Here is the answer for System.web.mail
MailMessage mail = new MailMessage();
mail.To.Add("to@domain.com");
mail.From = new MailAddress("from@domain.com");
mail.Subject = "Email using Gmail";
mail.Body = "";
mail.IsBodyHtml = true;
SmtpClient smtp = new SmtpClient();
smtp.Host = "smtp.gmail.com";
smtp.EnableSsl = true;
smtp.Credentials = new System.Net.NetworkCredential(mail.From,"YourPassword");
smtp.Send(mail);