It is a common scenario for any corporate environment to periodically send large quantities of e-mails.

However, as important as sending the mail - is knowing who didn't receive the mail. It is therefor vital to catch all e-mails that could not be delivered to a recipient. People from Sales love this sort of detail!

Luckily the System.Net.Mail namespace offers us the SmtpFailedRecipientException class which is ideal for this scenario.

The example below is a nearly complete blueprint for a mailingtool and demonstrates the use of this class. It will:

1. Send an e-mail to multiple recipients, one-by-one
2. Catch the ones that fail
3. Try to re-send failed deliveries that can be handled (i.e. MailboxBusy)
4. Leave you with a list of recipients that failed for unhandled reasons

static void Main(string[] args)

{

    // Generate a list of recipients:

    List<string> ListOfRecipients = new List<string>();

    ListOfRecipients.Add("johndoe@microsoft.com");

    ListOfRecipients.Add("johndoe@nonexistingdomain.com");    // this wil fail, logically

    // Generate a (empty) container for recipients that will fail ("recipient", "errormessage"):

    Dictionary<string, string> ListOfFailedRecipients = new Dictionary<string, string>();

      // Make up the variables for the e-mail (we'll re-use them in the exception handling):

    SmtpClient mailServer = new SmtpClient("smtp.yourisp.com");

    string mailSender = "ben@contoso.com";

    string mailSubject = "This is the subject";

    string mailBody = "Lorem ipsum dolor sit amet, consectetuer adipiscing elit...";

      foreach (string recipient in ListOfRecipients)

    {

        try

        {

            // Try to send an e-mail to each recipient:

            mailServer.Send(mailSender, recipient, mailSubject, mailBody);

        }

        catch (SmtpFailedRecipientException ex)

        {

            // Sending failed. Let's determine *why* and see if we can do anything about it:

            switch (ex.StatusCode)

            {

                case SmtpStatusCode.MailboxBusy:

                    // ... wait for a couple of seconds, then try again:

                    System.Threading.Thread.Sleep(3000);

                    mailServer.Send(mailSender, recipient, mailSubject, mailBody);

                    break;

                default:

                    // ... in any other case, just give up and mark the recipient as 'failed':

                    ListOfFailedRecipients.Add(recipient, ex.Message);

                    break;

            }

        }

        catch (Exception ex)

        {

            // Catch any other exception as a 'failed' delivery:

            ListOfFailedRecipients.Add(recipient, ex.Message);

        }

    }

}

At the end we naturally could do something with the ListOfFailedRecipients, considering we now have both the recipient and a specific error message on "why" the delivery failed.

Additional exception handling

But we could be more effective by extending the switch statement with more specific SmtpStatusCode's. Here is the complete list of possible status goodies to play around with:

Do note that although the list seems exhaustive, the SmtpStatusCode is based on the reply from the SMTP server. So, if your SMTP server (or ISP) implements the SMTP RFC 2821 standard, which most do, then you're good to go.

When you want to handle SMTP server exceptions you can always use the more generic SmtpException() class. Like the SmtpFailedRecipient and SmtpFailedRecipients classes mentioned above it also contains the StatusCode property, but is not limited to handling only message deliveries that failed. It also handles all sorts of (SMTP) server and connection, resulting in the fact that you cannot always point an error from SmtpException() to a single recipient.

Update - RecipientNotFound

It should be noted that the SmtpFailedRecipientException() only works for local mail servers and recipients. Think Exchange, ActiveDirectory etc. If you are sending to an external mail system it cannot be determined directly if the recipient's mailbox is available.

Although this may seem limiting, it is quite logic. Think of what spammers would do if this type of verification was available to them.

In order for mailings to give trustable results, make sure you send to people who actually gave their address to you and also make sure you provide a valid "from:" sender address. In the latter case, you will get the 550 errors related to "Recipient not available" bounced in your mailbox.