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

// ReplyTo property
message.ReplyTo = new MailAddress("reply@server.com", "ReplyTo Name");
// Priority property
message.Priority = MailPriority.High;

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

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