C# HttpWebRequest issue: The underlying connection was closed: An unexpected error occurred on a send

I am making a winforms apps, and I would like to send multiple HttpWebRequest to my Https API url at same time ( like 20-30 requests) and total request not limited. However after a random time, my Winforms app runs into issue

error: The underlying connection was closed: An unexpected error occurred on a send

Which parts of my code should I change to make my app run correctly and efficiently, and send multiple HttpWebRequest requests without issue

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12
_tasks = new List<Task>();
cancelSources = new List<CancellationTokenSource>();

foreach (var emailList in smalChunks)
{
    CancellationTokenSource cs = new CancellationTokenSource();
    Task task = Task.Run(async () =>
    {
        await HandleEmailList(emailList, userData, cs);
    }, cs.Token);

    _tasks.Add(task);
    cancelSources.Add(cs);
}
await Task.WhenAll(_tasks.ToArray()).ConfigureAwait(false);

The function bellow handle HttpWebRequest

private async Task HandleEmailList(List<SorterResults> emailList, CancellationTokenSource cs)
{
    try
    {
        ar sortUrl = $"http://localhost:5000/api/xxx";

        var httpWebRequest = (HttpWebRequest)WebRequest.Create(sortUrl);
        httpWebRequest.ContentType = "application/json";
        httpWebRequest.Method = "POST";

        using (var streamWriter = new StreamWriter(await httpWebRequest.GetRequestStreamAsync()))
        {
            var jsonData = JsonConvert.SerializeObject(emailList.ToArray());
            var mailData = $"{{\"mails\":{jsonData}}}";
            await streamWriter.WriteAsync(mailData);
        }

        APIResponseData responseResult;
        var httpResponse = (HttpWebResponse)(await httpWebRequest.GetResponseAsync());
        using (var streamReader = new StreamReader(httpResponse.GetResponseStream()))
        {
            var result = await streamReader.ReadToEndAsync();
            responseResult = JsonConvert.DeserializeObject<APIResponseData>(result);
        }
				
		cs.Token.ThrowIfCancellationRequested();
	}
	catch (Exception ex)
    {
        //ex.Message: The underlying connection was closed: An unexpected error occurred on a send
    }
}

Can you confirm if the internal task runs at all?
Stick a breakpoint on this line and see if it is ever hit:

cs.Token.ThrowIfCancellationRequested() 

I suspect it could be something like when your using block disposes of the stream writer, it also disposes the stream and this closes the connection. I'd change the exception settings in visual studio to break on whatever the exception type is, then you can interrogate this further.

Also, please tell me you aren't building an email botnet!

Here's some additional unsolicited advice:

Don't call _tasks.ToArray() when a method takes an IEnumerable, it's fine as a list and that translation just slows your application down

Why are you creating multiple CancellationTokenSource objects? Could you just create a single one, then pass the token down?

Why are you forwarding the task with an anonymous Task.Run delegate? You have a function that returns a task, so can't you just add that to the list?

With these changes:

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12
_tasks = new List<Task>();
CancellationTokenSource cs = new CancellationTokenSource();
    
foreach (var emailList in smalChunks)
{
    Task task = HandleEmailList(emailList, userData, cs);
    _tasks.Add(task);
}
await Task.WhenAll(_tasks).ConfigureAwait(false);

Also change the signature of the task function

private async Task HandleEmailList(List<SorterResults> emailList, CancellationToken ct)