Sending Exchange WS emails - Not enough free threads in the ThreadPool to complete the operation

I need to overcome this error:

There were not enough free threads in the ThreadPool to complete the operation.

I get it 80% of the time when queuing a bunch of Correspondence objects (with SSRS Reports) and sending them via Exchange WS. The reports are generated and emailed using a ThreadPool.

This is what the code looks like, it was built on .Net 2.0 hence no usage of TPL or Async/Await.

public static void QueueCorrespondence(ref Correspondence corr)
{
    Queue corrQ = CorrespondenceQueue(corr.Medium);
    // Create an AsyncOperation, using the CorrespondenceID as a unique id
    AsyncOperation asOp = AsyncOperationManager.CreateOperation(corr.CorrespondenceID);
    SendCorrespondencesInCurrentQueueArgs sendCorrespondencesInCurrentQueueArgs = new SendCorrespondencesInCurrentQueueArgs();
    sendCorrespondencesInCurrentQueueArgs.AsOpArg = asOp;
    sendCorrespondencesInCurrentQueueArgs.CorrQueueArg = corrQ;
    lock (_correspondenceQs) {
        if (corrQ.Count == 0) {
            corrQ.Enqueue(corr);
            ThreadPool.QueueUserWorkItem(new WaitCallback(SendCorrespondencesInCurrentQueue), sendCorrespondencesInCurrentQueueArgs);
        } else {
            corrQ.Enqueue(corr);
        }
    }
}

/// <summary>Do the processing necessary on the new thread</summary>
/// <param name="scicqa"></param>
/// <remarks>This runs on a background thread in the threadpool</remarks>
private static void SendCorrespondencesInCurrentQueue(SendCorrespondencesInCurrentQueueArgs scicqa)
{
    Correspondence corr = null;
    bool allReportsSuccessfullySent = false;
    //Dequeues the correspondence from the queue and returns it (ByRef)
    while (DequeueCorrespondence(ref corr, ref scicqa.CorrQueueArg)) {
        try {
            //This calls Exchange Web Services to send emails
            Send(corr);
            allReportsSuccessfullySent = true;
        } catch (Exception ex) {
            Incident.NewIncident("Unexpected Error", Incident.IncidentLevelEnum.ErrorLevel, ex, true, string.Format("Sending correspondence {0} via Medium {1}", corr.CorrespondenceID.ToString, corr.Medium));
        } finally {
            MailRoomArgs mra = new MailRoomArgs();
            mra.CorrArg = corr;
            mra.TransferSuccessArg = allReportsSuccessfullySent;
            //Success or not, call the RaiseCorrespondenceSentEvent subroutine via this delegate.
            scicqa.AsOpArg.Post(onCorrespondenceSentDelegate, mra);
        }
    }
}

/// <summary>Pull the next correspondence item off the queue, returning true if success</summary>
private static bool DequeueCorrespondence(ref Correspondence corr, ref Queue corrQ)
{
    lock (_correspondenceQs) {
        if (corrQ.Count > 0) {
            corr = corrQ.Dequeue;
            return true;
        } else {
            corr = null;
            return false;
        }
    }
}

Here is the stack trace. See the above functions call into Exchange WS and BeginGetRequestStream throws an exception due to lack of threads in the ThreadPool.

   at System.Net.HttpWebRequest.BeginGetRequestStream(AsyncCallback callback, Object state)
   at Microsoft.Exchange.WebServices.Data.EwsHttpWebRequest.Microsoft.Exchange.WebServices.Data.IEwsHttpWebRequest.BeginGetRequestStream(AsyncCallback callback, Object state)
   at Microsoft.Exchange.WebServices.Data.ServiceRequestBase.GetWebRequestStream(IEwsHttpWebRequest request)
   at Microsoft.Exchange.WebServices.Data.ServiceRequestBase.EmitRequest(IEwsHttpWebRequest request)
   at Microsoft.Exchange.WebServices.Data.ServiceRequestBase.BuildEwsHttpWebRequest()
   at Microsoft.Exchange.WebServices.Data.ServiceRequestBase.ValidateAndEmitRequest(IEwsHttpWebRequest& request)
   at Microsoft.Exchange.WebServices.Data.SimpleServiceRequestBase.InternalExecute()
   at Microsoft.Exchange.WebServices.Data.MultiResponseServiceRequest`1.Execute()
   at Microsoft.Exchange.WebServices.Data.ExchangeService.InternalCreateItems(IEnumerable`1 items, FolderId parentFolderId, Nullable`1 messageDisposition, Nullable`1 sendInvitationsMode, ServiceErrorHandling errorHandling)
   at Microsoft.Exchange.WebServices.Data.ExchangeService.CreateItem(Item item, FolderId parentFolderId, Nullable`1 messageDisposition, Nullable`1 sendInvitationsMode)
   at Microsoft.Exchange.WebServices.Data.Item.InternalCreate(FolderId parentFolderId, Nullable`1 messageDisposition, Nullable`1 sendInvitationsMode)
   at Microsoft.Exchange.WebServices.Data.Item.Save(WellKnownFolderName parentFolderName)
   at XYZ.WM.CIMS.BusinessObjects.ExchangeServer.SendCorrespondence(Correspondence& corr)
   at XYZ.WM.CIMS.BusinessObjects.Email.SendCorrespondence(Correspondence& corr)
   at XYZ.WM.CIMS.BusinessObjects.CorrespondenceMediumBase.Send(Correspondence& corr)
   at XYZ.WM.CIMS.BusinessObjects.CorrespondenceMediumBase.SendCorrespondencesInCurrentQueue(SendCorrespondencesInCurrentQueueArgs scicqa)

Sending emails on the main thread works fine. After a couple of days on this I'm pretty certain the problem could be avoided if I didn't call Exchange WS on a worker thread as shown below.

It doesn't matter if I Save, Send or SendAndSave emails I still get the error:

private  exchangeService = new ExchangeService(3); //Exchange2010 SP2
public void SendCorrespondence(ref Correspondence corr)
{
    exchangeService.Url = new Uri("http://ift.tt/1dpOkBc");
    exchangeService.UseDefaultCredentials = true;
    string[] emailAddresses = corr.SubscriptionObject.SubscriptionRecipients.EmailAddresses;

    EmailMessage message = default(EmailMessage);
    message = new EmailMessage(exchangeService);

    if (emailAddresses.Count > 1) {
        message.BccRecipients.Add(string.Join(RECIPIENTS_SEPARATOR, emailAddresses));
    } 
    else if (emailAddresses.Count == 1) {
        message.ToRecipients.Add(emailAddresses(0));
    }

    message.Subject = corr.SubscriptionObject.EmailSubject;
    EmailAddress fromSender = new EmailAddress();
    fromSender.Address = _instOpsSharedOutlookMailAccount;
    fromSender.Name = _instOpsSharedMailAccountName;
    fromSender.MailboxType = MailboxType.Mailbox;
    message.From = fromSender;

    foreach (ReportBase generatedReport in corr.GeneratedReports) {
        message.Attachments.AddFileAttachment(generatedReport.GeneratedDocument.FullName);
    }

    message.Body = new BodyType();
    message.Body.BodyType = BodyType.HTML;
    message.Body.Text = corr.SubscriptionObject.EmailTemplate;
    if (corr.SubscriptionObject.SendToDraft) {
        //Saving causes the problem !!!!!!!!!!!
    } else if (Convert.ToBoolean(Code.GetCodeTextValue(PARENT_CODE, "ExchangeSaveMsgOnSend", RETURN_DEFAULT_STRING_IF_NO_DATA, "True"))) {
        //Sending and Saving causes the problem !!!!!!!!!!!
        message.SendAndSaveCopy();      
    } else {
        //Sending causes the problem !!!!!!!!!!!
        message.Send(); 
    }
}

In fact using any Exchange methods, eg: ExchangeService.ResolveName(logonID) produces the error when called from a thread in the pool.

Does anyone know how I can send the emails without exhausting the ThreadPool? I don't want to re-architect this code too much, but would be happy to implement a simple way to avoid sending the email on a thread if anyone knows how.

The problem does not occur with Lotus Notes (yeah I know... lol!). I was using Exchange WS to get around the Outlook nag when an external program uses it to send emails. I would prefer not to use the Outlook Client but if there is no good solution I guess I will have to. Using Smtp client is not on the cards (as saving drafts in group folders is a major feature). I have also tried reducing the maximum number of threads in the ThreadPool but as I suspected that doesn't make a difference.

Any help would be much appreciated!!