May be a duplicate of https://github.com/localstack/localstack/issues/988 but due to the age I will re raise.

I'm having issues when I throw an exception in a lambda - it isn't appearing on my dead letter queue. Is there something I am missing, I would like an sqs event exception to appear on the poison queue when an exception happens.

Below is the basic config:
 ````
    sqsclient.create_queue(QueueName = 'test-queue')
    sqsclient.create_queue(QueueName = 'test-queue-poison')

    lambda_client.create_function(
            FunctionName='MyFunction',
            Runtime='dotnetcore2.1',
            Role='role',
            Handler='Handler',
            Code=code,
	    DeadLetterConfig={'TargetArn': 'test-queue-poison-arn'})

     lambda_client.create_event_source_mapping(
		EventSourceArn='test-queue-arn',
		FunctionName='MyFunction',
		Enabled=True,
		BatchSize=100,
		StartingPosition='LATEST')
````

So when I raise a new (.net)exception within the lambda function no message is going to the dead letter queue. I can confirm that all is attached together OK as all other scenarios work, and the above code is pseudo-ish.
result = {}
        for i in range(1, 500):
            key1 = 'Attribute.%s.Name' % i
            key2 = 'Attribute.%s.Value' % i
            if key1 not in req_data:
                break
            key_name = req_data[key1][0]
            key_value = req_data[key2][0]
            result[key_name] = key_value
        return result
    def _send_message(self, path, data, req_data, headers):
        queue_url = self._queue_url(path, req_data, headers)
        queue_name = queue_url[queue_url.rindex('/') + 1:]
        message_body = req_data.get('MessageBody', [None])[0]
        message_attributes = self.format_message_attributes(req_data)
        region_name = extract_region_from_auth_header(headers)
        process_result = lambda_api.process_sqs_message(message_body,
            message_attributes, queue_name, region_name=region_name)
        if process_result:
            
            new_response = Response()
            message_attr_hash = self.get_message_attributes_md5(req_data)
            new_response._content = SUCCESSFUL_SEND_MESSAGE_XML_TEMPLATE.format(
                message_attr_hash=message_attr_hash,
                message_body_hash=md5(message_body),
                message_id=str(uuid.uuid4())
            )
            new_response.status_code = 200
            return new_response
    def _set_queue_attributes(self, path, req_data, headers):