static void Send(string login, string password , string mailSend, string subject , string body , string proxyHost, int proxyPort)
{

    SmtpClient smtp = new SmtpClient("smtp.mail.ru", 587);
    smtp.Credentials = new System.Net.NetworkCredential(login, password);

    MailMessage mail = new MailMessage();

    mail.From = new MailAddress(login);
    mail.To.Add(new MailAddress(mailSend));
    mail.Subject = subject;
    mail.Body = body;
    smtp.EnableSsl = true;

    smtp.Send(mail);
}

var smtp = new SmtpConnection();
smtp.Proxy = new HttpProxy("ip-адрес", 8080);
smtp.Connect("smtp.mail.ru", 587);

smtp.ExtendedHello("mail.ru");
smtp.StartTls("mail.ru");

smtp.ExtendedHello("mail.ru");
smtp.AuthPlain("mail_acc", "password");

smtp.Mail("mail_acc@mail.ru");
smtp.Recipient("recipient@gmail.com");
smtp.Data(EmailFormatter.GetText("Bill Gates <mail_acc@mail.ru>", "Subject", "Steve Jobs <recipient@gmail.com>", "Hello"));
//smtp.Data(EmailFormatter.GetHtml("Bill Gates <mail_acc@mail.ru>", "Subject", "Steve Jobs <recipient@gmail.com>", "<h1>Hello!</h1><a href='stackoverflow.com'>click!</a>"));

smtp.Quit();