ThreadPool and Memory (BTStackServer) exception - .NET - asp.net

Question

In continuation with my problem here, I created a test app that uses ThreadPool threads to send emails. Here's the code I am using.
protected void btnSend_Click(object sender, EventArgs e)
{
EmailInfo em = new EmailInfo { Body = "Test", FromEmail = "test#test.com", Subject = "Test Email", To = "test#test.com" };
//txtNumEmails is a textbox where I can enter number of emails to send
for (int i = 0; i < Convert.ToInt32(this.txtNumEmails.Text); i++)
{
bool bResult = ThreadPool.QueueUserWorkItem(new WaitCallback(EmailNow), em);
}
}
public void EmailNow(object emailInfo) // Email Info object
{
EmailInfo em = emailInfo as EmailInfo;
SmtpClient client = new SmtpClient("localhost");
client.UseDefaultCredentials = true;
client.DeliveryMethod = SmtpDeliveryMethod.SpecifiedPickupDirectory;
client.PickupDirectoryLocation = #"C:\temp\testemails\";
MailMessage m = new MailMessage();
m.To.Add(new MailAddress(em.To));
m.Body = em.Body;
m.IsBodyHtml = false;
m.From = new MailAddress(em.FromEmail);
m.Subject = em.Subject;
client.Send(m);
}
For smaller numbers (10k, 50k) they work great, but once I increase the number to 200k (which is my target), I got this exception:
It created about 186k emails before it threw this exception.
I am assuming this is not the exception caused due to lack of disk space (since i am storing all the emails locally in C:\temp\testemails but because it was low in RAM(?). As suggested by a this user, I used semaphores to limit them to 10k, but still ran in to the same issue. Here's the code for the one with semaphores.
protected void Button1_Click(object sender, EventArgs e)
{
EmailInfo em = new EmailInfo { Body = "Test", FromEmail = "test#test.com", Subject = "Test Email", To = "test#test.com" };
var semaphore = new SemaphoreSlim(10000, 10000);
for (int i = 0; i < Convert.ToInt32(this.txtNumEmails.Text); i++)
{
semaphore.Wait();
bool bResult = ThreadPool.QueueUserWorkItem( (state) => {
try
{
EmailNow(em);
}
catch (Exception)
{
throw;
}
finally
{
semaphore.Release();
}
}, null);
}
}
This one threw an exception as well but I see all 200k emails in the folder. I can definitely use try catch and gracefully exit if this exception occurs, but how would I prevent this from happening in the first place.

Answer 1

Try disposing SmtpClient object by either wrapping inside using block or calling dispose() explicitly.

Answer 2

Try reducing the 10000 to 1000 for the SemaphoreSlim constructor.