Some emails are getting stuck in draft

Question

We are using Independentsoft's dll wrapper for webDAV to send email. If I send 20 identical emails some where between 1 and 5 emails will get as far as the drafts folder but not actually send. It'll send if I open the draft email in Outlook and then click send. Every-time I test it at least one email fails to send, sometimes more. Within the 20 emails it can vary which ones fail, 1st 2nd, 15th etc.

Here's the code:

foreach (var email in emailsToSend)
{
    try
    {
        var service = new Service("https://Ourserver/ews/Exchange.asmx", credentials);

        ItemId messageId = null;
        ItemInfoResponse sendResponse = null;

        if (!string.IsNullOrWhiteSpace(email.messageID))
        {
            try
            {
                messageId = new ItemId(email.messageID);

                sendResponse = service.Send(messageId);

                if (sendResponse.ResponseClass == ResponseClass.Success)
                {
                    email.Success = true;
                    email.SentTimeStamp = DateTime.Now;
                    email.messageID = messageId.Id;
                    return;
                }
                else
                {
                    email.LastError = sendResponse.Message;
                    email.messageID = null;
                }
            }
            catch(Exception ex)
            {
                email.LastError = ex.Message;
                email.messageID = null;
                email.ErrorCount++;
            }
            ctx.SubmitChanges();
        }

        var mm = new Message();
        foreach (var to in email.Tos)
        {
            switch (to.Type)
            {
                case 0: mm.ToRecipients.Add(new Mailbox(to.Address, to.Name));
                    break;
                case 1: mm.CcRecipients.Add(new Mailbox(to.Address, to.Name));
                    break;
                case 2: mm.BccRecipients.Add(new Mailbox(to.Address, to.Name));
                    break;
            }
        }

        var attachmentCollection = new List<Independentsoft.Exchange.Attachment>();

        foreach (var att in email.Attachments)
        {
            var attachment = new FileAttachment(att.FileData.ToArray(), att.FileName);
            attachmentCollection.Add(attachment);
        }

        var body = new Body(email.Body);
        body.Type = email.IsHTML ? BodyType.Html : BodyType.Best;
        mm.Body = body;
        mm.Subject = email.Subject;

        try
        {
            //check for use of secondary emailaddress
            var response = service.ResolveNames(string.Format("smtp:{0}", email.FromAddress));

            if (response.Resolutions.Count == 1 && response.Resolutions.Cast<Resolution>().First().Mailbox != null)
            {

                Mailbox userMailbox = response.Resolutions.Cast<Resolution>().First().Mailbox;
                StandardFolderId userInboxFolder = new StandardFolderId(StandardFolder.Inbox, userMailbox);

                Folder inbox = service.GetFolder(userInboxFolder);


                if (inbox != null)
                {
                    service.ExchangeImpersonation = new Identity(userMailbox.EmailAddress);
                }
                else
                {
                    mm.ReplyTo.Add(new Mailbox(email.FromAddress, email.FromName));
                }
            }
            else
            {
                mm.ReplyTo.Add(new Mailbox(email.FromAddress, email.FromName));
            }
        }
        catch
        {
            mm.ReplyTo.Add(new Mailbox(email.FromAddress, email.FromName));
        }

        messageId = service.CreateItem(mm);

        if (attachmentCollection.Count > 0)
        {
            IList<CreateAttachmentResponse> responses = service.CreateAttachment(attachmentCollection, messageId);
            messageId.ChangeKey = responses.Last().Attachment.AttachmentId.RootItemChangeKey;
        }

        sendResponse = service.Send(messageId);

        if (sendResponse.ResponseClass == ResponseClass.Success)
        {
            email.Success = true;
            email.SentTimeStamp = DateTime.Now;
            email.messageID = messageId.Id;
        }
        else
        {
            email.LastError = sendResponse.Message;
            email.messageID = messageId.Id;
            email.ErrorCount++;
        }
        ctx.SubmitChanges();


    }
    catch (Exception ex)
    {
        email.LastError = ex.Message;
        email.ErrorCount++;
        ctx.SubmitChanges();
    }
}

Once send from the drafts folder both successful and the unsuccessful email headers are almost identical.