これを試してください
using System.Net.Mail;
using System.Net;
var fromAddress = new MailAddress("from@gmail.com", "From Name");
var toAddress = new MailAddress("to@gmail.com", "To Name");
const string fromPassword = "password";
const string subject = "test";
const string body = "Hey now!!";
var smtp = new SmtpClient
{
Host = "smtp.gmail.com",
Port = 587,
EnableSsl = true,
DeliveryMethod = SmtpDeliveryMethod.Network,
Credentials = new NetworkCredential(fromAddress.Address, fromPassword),
Timeout = 20000
};
using (var message = new MailMessage(fromAddress, toAddress)
{
Subject = subject,
Body = body
})
{
smtp.Send(message);
}