[c#] smtpclient " failure sending mail"

here is my code

for(int i = 0; i < number ; i++)
{
    MailAddress to = new MailAddress(iMail.to);
    MailAddress from = new MailAddress(iMail.from, iMail.displayName);
    string body = iMail.body;
    string subject = iMail.sub;
    oMail = new MailMessage(from, to);
    oMail.Subject = subject;
    oMail.Body = body;
    oMail.IsBodyHtml = true;
    oMail.Priority = MailPriority.Normal;
    oMail.Sender = from;
    s = new SmtpClient(smtpServer);
    if (s != null)
    {
        s.Send(oMail);
    }
    oMail.Dispose();
    s = null;
}

this loops sends over 60,000 email. but my problem i am getting " failure sending mail" in some of the email some times 5000 and some time less then that rest of them gets delivered. and i have check all those error out email has valid email address. dont know what is the problem. i really need help in this.

Edit: This is my exception Trace

Error - Failure sending mail.; Inner Ex - System.IO.IOException: Unable to read data from the transport connection: net_io_connectionclosed. at System.Net.Mail.SmtpReplyReaderFactory.ProcessRead(Byte[] buffer, Int32 offset, Int32 read, Boolean readLine) at System.Net.Mail.SmtpReplyReaderFactory.ReadLines(SmtpReplyReader caller, Boolean oneLine) at System.Net.Mail.SmtpReplyReaderFactory.ReadLine(SmtpReplyReader caller) at System.Net.Mail.CheckCommand.Send(SmtpConnection conn, String& response) at System.Net.Mail.MailCommand.Send(SmtpConnection conn, Byte[] command, String from) at System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, SmtpFailedRecipientException& exception)

what error do you get is it a SmtpFailedrecipientException? if so you can check the innerexceptions list and view the StatusCode to get more information. the link below has some good information

MSDN

Edit for the new information

Thisis a problem with finding your SMTP server from what I can see, though you say that it only happens on some emails. Are you using more than one smtp server and if so maybe you can tract the issue down to one in particular, if not it may be that the speed/amount of emails you are sending is causing your smtp server some issue.

For us, everything was fine, emails are very small and not a lot of them are sent and sudently it gave this error. It appeared that a technicien installed ASTARO which was preventing email to be sent. and we were getting this error so yes the error is a bit cryptic but I hope this could help others.

I experienced the same issue when sending high volume email. Setting the deliveryMethod property to PickupDirectoryFromIis fixed it for me. Also don't create a new SmtpClient everytime.

Five years later (I hope this developer isn't still waiting for a fix to this..)

I had the same issue, caused by the same error: I was declaring the SmtpClient inside the loop.

The fix is simple - declare it once, outside the loop...

MailAddress mail = null;
SmtpClient client = new SmtpClient();
client.Port = 25;
client.EnableSsl = false;
client.DeliveryMethod = SmtpDeliveryMethod.Network;
client.UseDefaultCredentials = true;
client.Host = smtpAddress;       //  Enter your company's email server here!

for(int i = 0; i < number ; i++)
{
    mail = new MailMessage(iMail.from, iMail.to);
    mail.Subject = iMail.sub;
    mail.Body = iMail.body;
    mail.IsBodyHtml = true;
    mail.Priority = MailPriority.Normal;
    mail.Sender = from;
    client.Send(mail);
}
mail.Dispose();
client.Dispose();

Seeing your loop for sending emails and the error which you provided there is only solution.
Declare the mail object out of the loop and assign fromaddress out of the loop which you are using for sending mails. The fromaddress field is getting assigned again and again in the loop that is your problem.

Well, the "failure sending e-mail" should hopefully have a bit more detail. But there are a few things that could cause this.

1. Restrictions on the "From" address. If you are using different from addresses, some could be blocked by your SMTP service from being able to send.
2. Flood prevention on your SMTP service could be stopping the e-mails from going out.

Regardless if it is one of these or another error, you will want to look at the exception and inner exception to get a bit more detail.

This error can appear when the web server can't access the mail server. Make sure the web server can reach the mail server, for instance pinging it.