为了避免Gmail的安全问题，你应该先从Gmail的设置中生成一个应用程序密码，即使你使用两步验证，你也可以使用这个密码而不是真实密码来发送电子邮件。

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);

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。

这是发送邮件的附件。简单而简短。

来源:http://coding-issues.blogspot.in/2012/11/sending-email-with-attachments-from-c.html

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

public void email_send()
{
    MailMessage mail = new MailMessage();
    SmtpClient SmtpServer = new SmtpClient("smtp.gmail.com");
    mail.From = new MailAddress("your mail@gmail.com");
    mail.To.Add("to_mail@gmail.com");
    mail.Subject = "Test Mail - 1";
    mail.Body = "mail with attachment";

    System.Net.Mail.Attachment attachment;
    attachment = new System.Net.Mail.Attachment("c:/textfile.txt");
    mail.Attachments.Add(attachment);

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

    SmtpServer.Send(mail);

}

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);
    }
}

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();
     }
}
}
}