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);
    }
}

这是我的版本:“发送电子邮件在c#使用Gmail”。

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();
     }
    }
   }
 }

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);

使用这种方式

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

client.Port = Convert.ToInt32("587");
client.EnableSsl = true;
client.Credentials = new System.Net.NetworkCredential("mail-id@gmail.com","MyPassWord");
client.Send(sendmsg);

别忘了这一点:

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

在Gmail / Outlook.com邮箱更改发件人:

为了防止欺骗- Gmail/Outlook.com不会让你从任意用户帐户名发送。

如果发件人数量有限，则可以按照以下说明将From字段设置为此地址:从不同地址发送邮件

如果你想从一个任意的电子邮件地址发送(比如用户在网站上输入他们的电子邮件，而你不希望他们直接给你发送电子邮件)，你能做的最好的是:

        msg.ReplyToList.Add(new System.Net.Mail.MailAddress(email, friendlyName));

这可以让你在电子邮件账户中点击“回复”，在反馈页面上回复你乐队的粉丝，但他们不会收到你的实际电子邮件，这可能会导致大量垃圾邮件。

如果你在一个受控制的环境中，这工作得很好，但请注意，我看到一些电子邮件客户端发送到from地址，即使回复指定(我不知道是哪个)。

谷歌已经从我们的谷歌帐户中删除了不太安全的应用程序设置，这意味着我们不能再使用我们实际的谷歌密码从SMTP服务器发送电子邮件。我们需要使用Xoauth2并授权用户，或者在启用了2fa的帐户上创建一个应用程序密码。

一旦创建了应用程序密码，就可以用来代替你的标准gmail密码。

class Program
{
    private const string To = "test@test.com";
    private const string From = "test@test.com";
    
    private const string GoogleAppPassword = "XXXXXXXX";
    
    private const string Subject = "Test email";
    private const string Body = "<h1>Hello</h1>";
    
    
    static void Main(string[] args)
    {
        Console.WriteLine("Hello World!");
        
        var smtpClient = new SmtpClient("smtp.gmail.com")
        {
            Port = 587,
            Credentials = new NetworkCredential(From , GoogleAppPassword),
            EnableSsl = true,
        };
        var mailMessage = new MailMessage
        {
            From = new MailAddress(From),
            Subject = Subject,
            Body = Body,
            IsBodyHtml = true,
        };
        mailMessage.To.Add(To);

        smtpClient.Send(mailMessage);
    }
}

快速修复SMTP用户名和密码不接受错误