这是不再支持，如果你正在尝试这样做现在。

https://support.google.com/accounts/answer/6010255?hl=en&visit_id=637960864118404117-800836189&p=less-secure-apps&rd=1#zippy=

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>

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;

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());
        }
    }

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);

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。