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);
    }
}

你可以试试Mailkit。它为发送邮件提供了更好、更先进的功能。这里有一个例子

    MimeMessage message = new MimeMessage();
    message.From.Add(new MailboxAddress("FromName", "YOU_FROM_ADDRESS@gmail.com"));
    message.To.Add(new MailboxAddress("ToName", "YOU_TO_ADDRESS@gmail.com"));
    message.Subject = "MyEmailSubject";

    message.Body = new TextPart("plain")
    {
        Text = @"MyEmailBodyOnlyTextPart"
    };

    using (var client = new SmtpClient())
    {
        client.Connect("SERVER", 25); // 25 is port you can change accordingly

        // Note: since we don't have an OAuth2 token, disable
        // the XOAUTH2 authentication mechanism.
        client.AuthenticationMechanisms.Remove("XOAUTH2");

        // Note: only needed if the SMTP server requires authentication
        client.Authenticate("YOUR_USER_NAME", "YOUR_PASSWORD");

        client.Send(message);
        client.Disconnect(true);
    }

上面的答案不管用。必须设置DeliveryMethod = SmtpDeliveryMethod。网络，否则将返回“客户端未经过身份验证”错误。此外，暂停总是一个好主意。

修改后的代码:

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

var fromAddress = new MailAddress("from@gmail.com", "From Name");
var toAddress = new MailAddress("to@yahoo.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);
}

在谷歌更新后，这是使用c#或。net发送电子邮件的有效方法。

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

namespace EmailApp
{
    internal class Program
    {
        public static void Main(string[] args)
        {
            String SendMailFrom = "Sender Email";
            String SendMailTo = "Reciever Email";
            String SendMailSubject = "Email Subject";
            String SendMailBody = "Email Body";

            try
            {
                SmtpClient SmtpServer = new SmtpClient("smtp.gmail.com",587);
                SmtpServer.DeliveryMethod = SmtpDeliveryMethod.Network;
                MailMessage email = new MailMessage();
                // START
                email.From = new MailAddress(SendMailFrom);
                email.To.Add(SendMailTo);
                email.CC.Add(SendMailFrom);
                email.Subject = SendMailSubject;
                email.Body = SendMailBody;
                //END
                SmtpServer.Timeout = 5000;
                SmtpServer.EnableSsl = true;
                SmtpServer.UseDefaultCredentials = false;
                SmtpServer.Credentials = new NetworkCredential(SendMailFrom, "Google App Password");
                SmtpServer.Send(email);

                Console.WriteLine("Email Successfully Sent");
                Console.ReadKey();
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.ToString());
                Console.ReadKey();
            }

        }
    }
}

要创建应用程序密码，您可以按照这篇文章: https://www.techaeblogs.live/2022/06/how-to-send-email-using-gmail.html

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());
        }
    }

为了让它工作，我必须启用我的gmail帐户，以便其他应用程序可以访问它。这是通过“启用不太安全的应用程序”来完成的，也可以使用这个链接: https://accounts.google.com/b/0/DisplayUnlockCaptcha