smtpclient " failure sending mail"

Question

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)

Answer 1

apparently this problem got solved just by increasing queue size on my 3rd party smtp server. but the answer by Nip sounds like it is fairly usefull too