System.Web.Mail.MailMessage MyEmail = new System.Web.Mail.MailMessage();
MyEmail.To = txtTo.Text;
MyEmail.From = txtFrom.Text;
MyEmail.Body = rtbBody.Text.ToString;
MyEmail.Subject = txtObject.Text;
MyEmail.BodyFormat = Web.Mail.MailFormat.Text;
string SmtpServer;
SmtpServer = "smtp.miodominio.it";
MyEmail.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpserver", SmtpServer);
MyEmail.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpserverport", "25");
MyEmail.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendusing", "2");
MyEmail.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpauthenticate", "1");
MyEmail.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendusername", "username");
MyEmail.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendpassword", "password");
System.Web.Mail.SmtpMail.SmtpServer = SmtpServer;
System.Web.Mail.SmtpMail.Send(MyEmail);