.NET SmtpClient: Is there a way to make sure that all emails resolve prior to sending MailMessage?

问题

I am using SmtpClient to send an email to multiple recipients. As employees leave the company, their email addresses become invalid. Since their email addresses remain in our database until deleted manually, trying to send an email to them causes our application to throw an exception during SmtpClient.Send(MailMessage). However, in spite of the exception being thrown, it still sends the email. This is a problem because we want to handle this error by blocking the user's attempt to save the record and display a friendly message advising to delete any invalid associates from the database.

If there were a way to iterate through all the recipeients to make sure they're all valid, we can keep all emails from being sent until the user satisifes a set of conditions.

回答1:

Its a very old question, I don't know if you have got it solved.

As per MSDN: http://msdn.microsoft.com/en-us/library/swas0fwc(v=vs.100).aspx

When sending e-mail using Send to multiple recipients and the SMTP server accepts some recipients as valid and rejects others, Send sends e-mail to the accepted recipients and then a SmtpFailedRecipientsException is thrown. The exception will contain a listing of the recipients that were rejected.

This is an example of catching this exception taken from MSDN:

try {
    client.Send(message);
}
catch (SmtpFailedRecipientsException ex) {
    for (int i = 0; i < ex.InnerExceptions.Length; i++) {
        SmtpStatusCode status = ex.InnerExceptions[i].StatusCode;
        if (status == SmtpStatusCode.MailboxBusy || status == SmtpStatusCode.MailboxUnavailable) {
            Console.WriteLine("Delivery failed - retrying in 5 seconds.");
            System.Threading.Thread.Sleep(5000);
            client.Send(message);
        } 
        else {
            Console.WriteLine("Failed to deliver message to {0}", ex.InnerExceptions[i].FailedRecipient);
        }
    }
}

Complete example here: http://msdn.microsoft.com/en-us/library/system.net.mail.smtpfailedrecipientsexception.aspx?cs-save-lang=1&cs-lang=csharp#code-snippet-2

Internally the Send uses the statuscode returned from RCPT TO command to raise the appropriate exception.

Check the implementation for PrepareCommand in the RecipientCommand.Send method of smtpTransport.SendMail (This method is called internally by SmtpClient.Send). It uses RCPT TO to get the StatusCode which is then parsed in the CheckResponse method and accordingly the SmtpFailedRecipientsException is raised. However, VRFY and RCPT both are not very reliable because the mail servers tend to delay (throttle NDR) or swallow the response as an anti-spam measure.

回答2:

Have a look at the following: How to check if an email address exists without sending an email?

What you want to do is check that the e-mails exist before continuing to send.

Therefore, as is stated in the linked answer, try to see if VRFY or RCPT are supported by your company mail server.

Quoting:

You can connect to the server, and issue a VRFY command. Very few servers support this command, but it is intended for exactly this. If the server responds with a 2.0.0 DSN, the user exists.

VRFY user

You can issue a RCPT, and see if the mail is rejected.

MAIL FROM:<>

RCPT TO:

来源：https://stackoverflow.com/questions/13274005/net-smtpclient-is-there-a-way-to-make-sure-that-all-emails-resolve-prior-to-se