Why I use google 'smtp' cannot send out email?

I'm not sure what is causing your problem. Here is some code I have been using to successfully send email through a gmail account:

const string from = "...";
var fromAddr = new MailAddress(from, "Bug Tracker");
var toAddr = new MailAddress("...@...", "...");
var client = new SmtpClient {
    Host = "smtp.gmail.com",
    Port = 587,
    EnableSsl = true,
    DeliveryMethod = SmtpDeliveryMethod.Network,
    UseDefaultCredentials = false,
    Timeout = 30 * 1000,
    Credentials = new NetworkCredential(fromAddr.Address, "...")
};
using (var msg = new MailMessage(fromAddr, toAddr)) {
    msg.Subject = "...";
    msg.Body = string.Format("username: {0}\nversion: {1}\n\n{2}", Environment.UserName, Assembly.GetExecutingAssembly().GetName().Version.ToString(3), cbtext);
    client.Send(msg);
}

I had the code that Ferruccio posted and this recently stopped working. I moved my settings into the .config file for my site and it started to work again:

<system.net>
    <mailSettings>
        <smtp from="fromEmail" deliveryMethod="Network">
            <network defaultCredentials="false" enableSsl="true" host="smtp.gmail.com" port="587"
                 userName="fromEmail" password="password"/>
        </smtp>
    </mailSettings>
</system.net>