セキュリティのためにSSLを有効にするために「smtp.EnableSsl=true」を使用できます。
MailMessage mail = new MailMessage();
mail.To.Add("" + to + "");
mail.From = new MailAddress("" + from + "");
mail.Subject = "Email using Gmail";
string Body = "Hi, this mail is to test sending mail" +
"";
mail.Body = Body;
mail.IsBodyHtml = true;
Attachment at = new Attachment(Server.MapPath("~/ExcelFile/TestCSV.csv"));
mail.Attachments.Add(at);
mail.Priority = MailPriority.High;
SmtpClient smtp = new SmtpClient();
smtp.Host = "smtp.gmail.com"; //Or Your SMTP Server Address
smtp.Credentials = new System.Net.NetworkCredential(""+ username +"", ""+ password +"");
smtp.EnableSsl = true;
smtp.Port = 587;
smtp.Send(mail);