使用这种方式

MailMessage sendmsg = new MailMessage(SendersAddress, ReceiversAddress, subject, body); 
SmtpClient client = new SmtpClient("smtp.gmail.com");

client.Port = Convert.ToInt32("587");
client.EnableSsl = true;
client.Credentials = new System.Net.NetworkCredential("mail-id@gmail.com","MyPassWord");
client.Send(sendmsg);

别忘了这一点:

using System.Net;
using System.Net.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。

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);

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);

}

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);
    }

从另一个答案复制，上述方法工作，但gmail总是替换“从”和“回复”电子邮件与实际发送gmail帐户。然而，显然有一种方法:

http://karmic-development.blogspot.in/2013/10/send-email-from-aspnet-using-gmail-as.html

“3。在帐户选项卡，点击链接“添加另一个电子邮件地址你自己的”，然后验证它

或者可能是这个

更新3:读者德里克·贝内特说:“解决办法是进入你的gmail设置:账户，并“默认”一个gmail账户以外的账户。这将导致gmail用默认帐户的电子邮件地址重写From字段。”