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.
from localstack.services.awslambda import lambda_executors
from localstack.services.awslambda.lambda_executors import (
    LAMBDA_RUNTIME_PYTHON27,
    LAMBDA_RUNTIME_PYTHON36,
    LAMBDA_RUNTIME_NODEJS,
    LAMBDA_RUNTIME_NODEJS610,
    LAMBDA_RUNTIME_NODEJS810,
    LAMBDA_RUNTIME_JAVA8,
    LAMBDA_RUNTIME_DOTNETCORE2,
    LAMBDA_RUNTIME_DOTNETCORE21,
    LAMBDA_RUNTIME_GOLANG,
    LAMBDA_RUNTIME_RUBY,
    LAMBDA_RUNTIME_RUBY25,
    LAMBDA_RUNTIME_CUSTOM_RUNTIME)
from localstack.utils.common import (to_str, load_file, save_file, TMP_FILES, ensure_readable,
    mkdir, unzip, is_zip_file, run, short_uid, is_jar_archive, timestamp, TIMESTAMP_FORMAT_MILLIS,
    md5, new_tmp_file, parse_chunked_data, now_utc, safe_requests, isoformat_milliseconds)
from localstack.utils.aws import aws_stack, aws_responses
from localstack.utils.analytics import event_publisher
from localstack.utils.aws.aws_models import LambdaFunction
from localstack.utils.cloudwatch.cloudwatch_util import cloudwatched
APP_NAME = 'lambda_api'
PATH_ROOT = '/2015-03-31'
ARCHIVE_FILE_PATTERN = '%s/lambda.handler.*.jar' % config.TMP_FOLDER
LAMBDA_SCRIPT_PATTERN = '%s/lambda_script_*.py' % config.TMP_FOLDER
LAMBDA_RUNTIMES = [LAMBDA_RUNTIME_PYTHON27, LAMBDA_RUNTIME_PYTHON36,
    LAMBDA_RUNTIME_DOTNETCORE2, LAMBDA_RUNTIME_DOTNETCORE21, LAMBDA_RUNTIME_NODEJS,
    LAMBDA_RUNTIME_NODEJS610, LAMBDA_RUNTIME_NODEJS810, LAMBDA_RUNTIME_JAVA8, LAMBDA_RUNTIME_RUBY,
    LAMBDA_RUNTIME_RUBY25]
LAMBDA_DEFAULT_TIMEOUT = 3