我们如何使用asp.net 确定电子邮件地址是否真的存在？

Question

We can validate an email address with a regular expression in asp.net. Now, how can we find that an email address really exists or not?

For example, farzaneh@yahoo.com has correct email format but does not exist.

This code only works with the Gmail SMTP:

protected void btnCheck_Click(object sender, EventArgs e)
{
    TcpClient tClient = new TcpClient("gmail-smtp-in.l.google.com", 25);

    string CRLF = "\r\n";
    byte[] dataBuffer;
    string ResponseString;
    NetworkStream netStream = tClient.GetStream();
    StreamReader reader = new StreamReader(netStream);
    ResponseString = reader.ReadLine();
    /* Perform HELO to SMTP Server and get Response */
    dataBuffer = BytesFromString("HELO KirtanHere" + CRLF);
    netStream.Write(dataBuffer, 0, dataBuffer.Length);
    ResponseString = reader.ReadLine();
    dataBuffer = BytesFromString("MAIL FROM:<YourGmailIDHere@gmail.com>" + CRLF);
    netStream.Write(dataBuffer, 0, dataBuffer.Length);
    ResponseString = reader.ReadLine();
    /* Read Response of the RCPT TO Message to know from google if it exist or not */
    dataBuffer = BytesFromString("RCPT TO:<" + TextBox1.Text.Trim() + ">" + CRLF);
    netStream.Write(dataBuffer, 0, dataBuffer.Length);
    ResponseString = reader.ReadLine();
    if (GetResponseCode(ResponseString) == 550)
    {
        Response.Write("Mai Address Does not Exist !<br/><br/>");
        Response.Write("<B><font color='red'>Original Error from Smtp Server :</font></b>" + ResponseString);
    }
    /* QUITE CONNECTION */
    dataBuffer = BytesFromString("QUITE" + CRLF);
    netStream.Write(dataBuffer, 0, dataBuffer.Length);
    tClient.Close();
}

Answer 1

I don't think this is possible because most email applications have firewalls or something like that to prevent spam attacks.

Even spam mails add a photo which is located on their site so they can determine if the photo is opened on your email address or not.

The best way to validate an email address is to send a validation code or link to the user which they have to confirm.

Answer 2

I have not done myself but I can give you a lead.

You can use MX record check of the domain first. Second check through handshake with SMTP if it validate xxx@google.com however what I recall it comes down to the server if it allows you or not.

try this link http://www.codeproject.com/Articles/5189/End-to-end-Email-Address-Verification-for-Applicat

Answer 3

If your website is integrated with an identity provider, you need to consider that normally an Identity Provider like Google, Yahoo don't share that information unless the user authorizes the third party to do it. You normally get redirected to the website(google, yahoo to be authenticated using username, email, or any other criteria that the identity provider manages), as soon as you are authorized, depending on the claims that they share with your application you can read from your authorization token, the email address. Hope this helps

Answer 4

It is not really possible: some mail provider won't acknowledge an adress doesn't exist to protect existing email adresses from spammers.

For example the VRFY and EXPN commands of the SMTP protocol (RFC 2821) are either ignored or error on most servers. Although it could be useful, the toxic behavior of spammers render the feature unusable.

Half-baked solution: send a mail. If it bounces, it doesn't exist. If it doesn't bounce, it exists or the mail server doesn't want you to know it doesn't exist for security reasons, and you can't really do anything about it apart from having some way to get the user to answer (link in mail, image in html mail for users not using text-only clients or/and who always download included images).