从2022年6月1日起，谷歌增加了一些安全功能

谷歌不再支持使用仅要求您使用您的用户名和密码登录您的谷歌帐户或直接使用谷歌帐户的用户名和密码发送邮件的第三方应用程序或设备。但是您仍然可以通过您的gmail帐户使用生成应用程序密码发送电子邮件。

以下是生成新密码的步骤。

访问https://myaccount.google.com/security 打开两步验证。 如有需要，请电话确认。 点击“应用程序密码”，就在“2步验证”勾的下方。请求邮件应用程序的新密码。

现在我们要用这个密码来发邮件，而不是你原来的账号密码。

下面是发送邮件的示例代码

public static void SendMailFromApp(string SMTPServer, int SMTP_Port, string From, string Password, string To, string Subject, string Body) {            
            var smtpClient = new SmtpClient(SMTPServer, SMTP_Port) {
                DeliveryMethod = SmtpDeliveryMethod.Network,
                UseDefaultCredentials = false,
                EnableSsl = true
            };                
            smtpClient.Credentials = new NetworkCredential(From, Password); //Use the new password, generated from google!
            var message = new System.Net.Mail.MailMessage(new System.Net.Mail.MailAddress(From, "SendMail2Step"), new System.Net.Mail.MailAddress(To, To));
            smtpClient.Send(message);
    }

你可以像下面这样调用方法

SendMailFromApp("smtp.gmail.com", 25, "mygmailaccount@gmail.com",
          "tyugyyj1556jhghg",//This will be generated by google, copy it here.
          "mailme@gmail.com", "New Mail Subject", "Body of mail from My App");

试试这个,

    private void button1_Click(object sender, EventArgs e)
    {
        try
        {
            MailMessage mail = new MailMessage();
            SmtpClient SmtpServer = new SmtpClient("smtp.gmail.com");

            mail.From = new MailAddress("your_email_address@gmail.com");
            mail.To.Add("to_address");
            mail.Subject = "Test Mail";
            mail.Body = "This is for testing SMTP mail from GMAIL";

            SmtpServer.Port = 587;
            SmtpServer.Credentials = new System.Net.NetworkCredential("username", "password");
            SmtpServer.EnableSsl = true;

            SmtpServer.Send(mail);
            MessageBox.Show("mail Send");
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.ToString());
        }
    }

一定要使用System.Net。邮件，而不是已弃用的System.Web.Mail。使用System.Web.Mail进行SSL是一堆拙劣的扩展。

using System.Net;
using System.Net.Mail;

var fromAddress = new MailAddress("from@gmail.com", "From Name");
var toAddress = new MailAddress("to@example.com", "To Name");
const string fromPassword = "fromPassword";
const string subject = "Subject";
const string body = "Body";

var smtp = new SmtpClient
{
    Host = "smtp.gmail.com",
    Port = 587,
    EnableSsl = true,
    DeliveryMethod = SmtpDeliveryMethod.Network,
    UseDefaultCredentials = false,
    Credentials = new NetworkCredential(fromAddress.Address, fromPassword)
};
using (var message = new MailMessage(fromAddress, toAddress)
{
    Subject = subject,
    Body = body
})
{
    smtp.Send(message);
}

此外，转到谷歌帐户>安全页面，并查看登录谷歌> 2步验证设置。

如果启用了，那么您必须生成一个密码，允许. net绕过2步验证。点击“Signing in To谷歌> App passwords”，选择App = Mail, device = Windows Computer，最后生成密码。在fromPassword常量中使用生成的密码，而不是标准的Gmail密码。 如果它被禁用，那么你必须打开Less secure app access，这是不推荐的!所以最好启用两步验证。

包括这个,

using System.Net.Mail;

然后,

MailMessage sendmsg = new MailMessage(SendersAddress, ReceiversAddress, subject, body); 
SmtpClient client = new SmtpClient("smtp.gmail.com");

client.Port = 587;
client.Credentials = new System.Net.NetworkCredential("mail-id@gmail.com","password");
client.EnableSsl = true;

client.Send(sendmsg);

using System;
using System.Net;
using System.Net.Mail;

namespace SendMailViaGmail
{
   class Program
   {
   static void Main(string[] args)
   {

      //Specify senders gmail address
      string SendersAddress = "Sendersaddress@gmail.com";
      //Specify The Address You want to sent Email To(can be any valid email address)
      string ReceiversAddress = "ReceiversAddress@yahoo.com";
      //Specify The password of gmial account u are using to sent mail(pw of sender@gmail.com)
      const string SendersPassword = "Password";
      //Write the subject of ur mail
      const string subject = "Testing";
      //Write the contents of your mail
      const string body = "Hi This Is my Mail From Gmail";

      try
      {
        //we will use Smtp client which allows us to send email using SMTP Protocol
        //i have specified the properties of SmtpClient smtp within{}
        //gmails smtp server name is smtp.gmail.com and port number is 587
        SmtpClient smtp = new SmtpClient
        {
           Host = "smtp.gmail.com",
           Port = 587,
           EnableSsl = true,
           DeliveryMethod = SmtpDeliveryMethod.Network,
           Credentials = new NetworkCredential(SendersAddress, SendersPassword),
           Timeout = 3000
        };

        //MailMessage represents a mail message
        //it is 4 parameters(From,TO,subject,body)

        MailMessage message = new MailMessage(SendersAddress, ReceiversAddress, subject, body);
        /*WE use smtp sever we specified above to send the message(MailMessage message)*/

        smtp.Send(message);
        Console.WriteLine("Message Sent Successfully");
        Console.ReadKey();
     }
     catch (Exception ex)
     {
        Console.WriteLine(ex.Message);
        Console.ReadKey();
     }
}
}
}

谷歌可能会阻止一些不使用现代安全标准的应用程序或设备的登录尝试。由于这些应用程序和设备更容易被侵入，屏蔽它们有助于让你的账户更安全。

不支持最新安全标准的应用程序包括:

iOS 6或以下版本的iPhone或iPad上的邮件应用程序 8.1版本之前Windows手机上的邮件应用程序 一些桌面邮件客户端，如Microsoft Outlook和Mozilla Thunderbird

因此，您必须在您的谷歌帐户中启用“较少安全登录”。

登录谷歌账号后，进入:

https://myaccount.google.com/lesssecureapps 或 https://www.google.com/settings/security/lesssecureapps

在c#中，你可以使用以下代码:

using (MailMessage mail = new MailMessage())
{
    mail.From = new MailAddress("email@gmail.com");
    mail.To.Add("somebody@domain.com");
    mail.Subject = "Hello World";
    mail.Body = "<h1>Hello</h1>";
    mail.IsBodyHtml = true;
    mail.Attachments.Add(new Attachment("C:\\file.zip"));

    using (SmtpClient smtp = new SmtpClient("smtp.gmail.com", 587))
    {
        smtp.Credentials = new NetworkCredential("email@gmail.com", "password");
        smtp.EnableSsl = true;
        smtp.Send(mail);
    }
}