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);

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);

}

上面的答案不管用。必须设置DeliveryMethod = SmtpDeliveryMethod。网络，否则将返回“客户端未经过身份验证”错误。此外，暂停总是一个好主意。

修改后的代码:

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

var fromAddress = new MailAddress("from@gmail.com", "From Name");
var toAddress = new MailAddress("to@yahoo.com", "To Name");
const string fromPassword = "password";
const string subject = "test";
const string body = "Hey now!!";

var smtp = new SmtpClient
{
    Host = "smtp.gmail.com",
    Port = 587,
    EnableSsl = true,
    DeliveryMethod = SmtpDeliveryMethod.Network,
    Credentials = new NetworkCredential(fromAddress.Address, fromPassword),
    Timeout = 20000
};
using (var message = new MailMessage(fromAddress, toAddress)
{
    Subject = subject,
    Body = body
})
{
    smtp.Send(message);
}

为了让它工作，我必须启用我的gmail帐户，以便其他应用程序可以访问它。这是通过“启用不太安全的应用程序”来完成的，也可以使用这个链接: https://accounts.google.com/b/0/DisplayUnlockCaptcha

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);

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);
    }