Zappa/AWS - Emails won't send and just timeout - django

Question

Currently, I have tried both plain-old Django SMTP and a few different api-based Django libraries for my transactional email provider (Postmark).
When I run my development server, everything works perfectly. Emails send via the Postmark API with no problem.
When I deploy to AWS with Zappa, visit my website, and do a task that is supposed to send an email (Ex. Resetting a user's password) the page continually loads until it says Endpoint request timed out.
I have tried setting the timeout of my AWS Lambda function to a longer duration in case Django decides to throw an error.
Here is the error that was thrown. Just keep in mind this error only happens in production. I created a custom management command in able to retrieve this error.
HTTPSConnectionPool(host='api.postmarkapp.com', port=443): Max retries exceeded with url: /email/batch (Caused by NewConnectionError('<urllib3.connection.VerifiedHTTPSConnection object at 0x7f6cfbd5dd30>: Failed to establish a new connection: [Errno 110] Connection timed out',)): ConnectionError
Traceback (most recent call last):
File "/var/task/handler.py", line 509, in lambda_handler
return LambdaHandler.lambda_handler(event, context)
File "/var/task/handler.py", line 240, in lambda_handler
return handler.handler(event, context)
File "/var/task/handler.py", line 376, in handler
management.call_command(*event['manage'].split(' '))
File "/var/task/django/core/management/__init__.py", line 131, in call_command
return command.execute(*args, **defaults)
File "/var/task/django/core/management/base.py", line 330, in execute
output = self.handle(*args, **options)
File "/var/task/users/management/commands/sendemail.py", line 13, in handle
fail_silently=False,
File "/var/task/django/core/mail/__init__.py", line 62, in send_mail
return mail.send()
File "/var/task/django/core/mail/message.py", line 348, in send
return self.get_connection(fail_silently).send_messages([self])
File "/var/task/postmarker/django/backend.py", line 66, in send_messages
responses = self.client.emails.send_batch(*prepared_messages, TrackOpens=self.get_option('TRACK_OPENS'))
File "/var/task/postmarker/models/emails.py", line 332, in send_batch
return self.EmailBatch(*emails).send(**extra)
File "/var/task/postmarker/models/emails.py", line 247, in send
responses = [self._manager._send_batch(*batch) for batch in chunks(emails, self.MAX_SIZE)]
File "/var/task/postmarker/models/emails.py", line 247, in <listcomp>
responses = [self._manager._send_batch(*batch) for batch in chunks(emails, self.MAX_SIZE)]
File "/var/task/postmarker/models/emails.py", line 276, in _send_batch
return self.call('POST', '/email/batch', data=emails)
File "/var/task/postmarker/models/base.py", line 72, in call
return self.client.call(*args, **kwargs)
File "/var/task/postmarker/core.py", line 106, in call
**kwargs
File "/var/task/postmarker/core.py", line 129, in _call
method, url, json=data, params=kwargs, headers=default_headers, timeout=self.timeout
File "/var/task/requests/sessions.py", line 508, in request
resp = self.send(prep, **send_kwargs)
File "/var/task/requests/sessions.py", line 618, in send
r = adapter.send(request, **kwargs)
File "/var/task/requests/adapters.py", line 508, in send
raise ConnectionError(e, request=request)
requests.exceptions.ConnectionError: HTTPSConnectionPool(host='api.postmarkapp.com', port=443): Max retries exceeded with url: /email/batch (Caused by NewConnectionError('<urllib3.connection.VerifiedHTTPSConnection object at 0x7f6cfbd5dd30>: Failed to establish a new connection: [Errno 110] Connection timed out',))
I have allowed all incoming and outgoing traffic to my AWS security group in an attempt to fix this. Still to no avail.
Any help would be greatly, greatly appreciated. Cheers.

Answer 1

The explanation is simple: A Lambda instance running in a VPC cannot access the internet:
When you add VPC configuration to a Lambda function, it can only access resources in that VPC. If a Lambda function needs to access both VPC resources and the public Internet, the VPC needs to have a Network Address Translation (NAT) instance inside the VPC.
The solution is also simple, if annoying: run a NAT Instance or NAT Gateway in the VPC. (An alternate solution is to take your Lambda out of the VPC, but that is a much bigger change.)
I am running Django / Zappa in Lambda with a NAT instance for connecting to Amazon Simple Email Service and it works fine.