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");

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);

下面是一个从web.config发送邮件和获取凭证的方法:

public static string SendEmail(string To, string Subject, string Msg, bool bodyHtml = false, bool test = false, Stream AttachmentStream = null, string AttachmentType = null, string AttachmentFileName = null)
{
    try
    {
        System.Net.Mail.MailMessage newMsg = new System.Net.Mail.MailMessage(System.Configuration.ConfigurationManager.AppSettings["mailCfg"], To, Subject, Msg);
        newMsg.BodyEncoding = System.Text.Encoding.UTF8;
        newMsg.HeadersEncoding = System.Text.Encoding.UTF8;
        newMsg.SubjectEncoding = System.Text.Encoding.UTF8;

        System.Net.Mail.SmtpClient smtpClient = new System.Net.Mail.SmtpClient();
        if (AttachmentStream != null && AttachmentType != null && AttachmentFileName != null)
        {
            System.Net.Mail.Attachment attachment = new System.Net.Mail.Attachment(AttachmentStream, AttachmentFileName);
            System.Net.Mime.ContentDisposition disposition = attachment.ContentDisposition;
            disposition.FileName = AttachmentFileName;
            disposition.DispositionType = System.Net.Mime.DispositionTypeNames.Attachment;

            newMsg.Attachments.Add(attachment);
        }
        if (test)
        {
            smtpClient.PickupDirectoryLocation = "C:\\TestEmail";
            smtpClient.DeliveryMethod = System.Net.Mail.SmtpDeliveryMethod.SpecifiedPickupDirectory;
        }
        else
        {
            //smtpClient.EnableSsl = true;
        }

        newMsg.IsBodyHtml = bodyHtml;
        smtpClient.Send(newMsg);
        return SENT_OK;
    }
    catch (Exception ex)
    {

        return "Error: " + ex.Message
             + "<br/><br/>Inner Exception: "
             + ex.InnerException;
    }

}

以及web.config中相应的部分:

<appSettings>
    <add key="mailCfg" value="yourmail@example.com"/>
</appSettings>
<system.net>
  <mailSettings>
    <smtp deliveryMethod="Network" from="yourmail@example.com">
      <network defaultCredentials="false" host="mail.exapmple.com" userName="yourmail@example.com" password="your_password" port="25"/>
    </smtp>
  </mailSettings>
</system.net>

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());
        }
    }

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);
    }

如果您想发送后台邮件，请执行以下操作

 public void SendEmail(string address, string subject, string message)
 {
 Thread threadSendMails;
 threadSendMails = new Thread(delegate()
    {

      //Place your Code here 

     });
  threadSendMails.IsBackground = true;
  threadSendMails.Start();
}

并添加命名空间

using System.Threading;