MailMessage message = new MailMessage();
message.From = new MailAddress("from@server.com", "From Name");
message.To.Add(new MailAddress("to@server.com", "To Name"));

// Add Custom Header
message.Headers.Add("CustomHeader", "value");
// Request a Read Receipt
message.Headers.Add("Disposition-Notification-To", "email@server.com");

message.Subject = "Subject";
message.Body = "This is a plain text";

SmtpClient emailClient = new SmtpClient("mail.server.com");
emailClient.Send(message);