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;

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");

我希望这段代码可以正常工作。你可以试试。

// Include this.                
using System.Net.Mail;

string fromAddress = "xyz@gmail.com";
string mailPassword = "*****";       // Mail id password from where mail will be sent.
string messageBody = "Write the body of the message here.";


// Create smtp connection.
SmtpClient client = new SmtpClient();
client.Port = 587;//outgoing port for the mail.
client.Host = "smtp.gmail.com";
client.EnableSsl = true;
client.Timeout = 10000;
client.DeliveryMethod = SmtpDeliveryMethod.Network;
client.UseDefaultCredentials = false;
client.Credentials = new System.Net.NetworkCredential(fromAddress, mailPassword);


// Fill the mail form.
var send_mail = new MailMessage();

send_mail.IsBodyHtml = true;
//address from where mail will be sent.
send_mail.From = new MailAddress("from@gmail.com");
//address to which mail will be sent.           
send_mail.To.Add(new MailAddress("to@example.com");
//subject of the mail.
send_mail.Subject = "put any subject here";

send_mail.Body = messageBody;
client.Send(send_mail);

如果你的谷歌密码不起作用，你可能需要在谷歌上为Gmail创建一个特定于应用程序的密码。 https://support.google.com/accounts/answer/185833?hl=en

编辑2022 从2022年5月30日起，谷歌将不再支持仅使用用户名和密码登录谷歌账户的第三方应用程序或设备。 但是你仍然可以通过你的gmail账户发送电子邮件。

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。

现在只需使用这个密码，而不是原来的一个为您的帐户!

public static void SendMail2Step(string SMTPServer, int SMTP_Port, string From, string Password, string To, string Subject, string Body, string[] FileNames) {            
            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);
    }

像这样使用:

SendMail2Step("smtp.gmail.com", 587, "youraccount@gmail.com",
          "yjkjcipfdfkytgqv",//This will be generated by google, copy it here.
          "recipient@barcodes.bg", "test message subject", "Test message body ...", null);

对于其他答案工作“从服务器”首先打开访问gmail帐户不太安全的应用程序。这将在2022年5月30日被弃用

看来最近谷歌改变了安全策略。评分最高的答案不再有效，直到您更改您的帐户设置如下所述:https://support.google.com/accounts/answer/6010255?hl=en-GB 2016年3月，谷歌再次更改设置地点!

试试这个

public static bool Send(string receiverEmail, string ReceiverName, string subject, string body)
{
        MailMessage mailMessage = new MailMessage();
        MailAddress mailAddress = new MailAddress("abc@gmail.com", "Sender Name"); // abc@gmail.com = input Sender Email Address 
        mailMessage.From = mailAddress;
        mailAddress = new MailAddress(receiverEmail, ReceiverName);
        mailMessage.To.Add(mailAddress);
        mailMessage.Subject = subject;
        mailMessage.Body = body;
        mailMessage.IsBodyHtml = true;

        SmtpClient mailSender = new SmtpClient("smtp.gmail.com", 587)
        {
            EnableSsl = true,
            UseDefaultCredentials = false,
            DeliveryMethod = System.Net.Mail.SmtpDeliveryMethod.Network,
            Credentials = new NetworkCredential("abc@gmail.com", "pass")   // abc@gmail.com = input sender email address  
                                                                           //pass = sender email password
        };

        try
        {
            mailSender.Send(mailMessage);
            return true;
        }
        catch (SmtpFailedRecipientException ex)
        { 
          // Write the exception to a Log file.
        }
        catch (SmtpException ex)
        { 
           // Write the exception to a Log file.
        }
        finally
        {
            mailSender = null;
            mailMessage.Dispose();
        }
        return false;
}