Fix message threading when using Amazon SES as a mail provider.
Review Request #2199 — Created July 28, 2021 and submitted
Amazon SES has some non-standard behavior around Message IDs. Normally, the sender (or an intermediary) will specify a Message ID for tracking and `In-Reply-To` headers, which can be used for bounce management or automated threading. We depend on this for our threading model, storing our generated Message IDs and attaching them to any replies. SES overrides any provided Message ID, which impacts our threading. They do provide a mechanism to get part of the overridden Message ID, by using a non-standard addition to the SMTP spec, which is to grab the final `250 Ok` after sending the e-mail and parsing out what's currently a UUID after the `Ok`. Python's `smtplib` does not support getting this information. Nor does Django's e-mail code. In order to access it, we provide our own SMTP backend, which is a subclass of Python's, and override the method used to get the result of sending an e-mail and store that for later retrieval. We then subclass Django's backend and pull out this result. If the SMTP hostname is an SES endpoint, we parse out the region and activate post-sending logic, pulling in that reply, checking if it appears to have the first part of a Message ID (the UUID), and then build on the domain (which will be `email.amazonses.com` for us-east-1, or `<region>.amazonses.com` for anything else) and store it back in the message, where it can be retrieved. This works for now, though as Amazon's behavior is non-standard and they don't offer official guidance, it's always possible this logic will need to be adjusted down the road. For anything other than SES, the new backend should not introduce any new behavior. Testing Done: Unit tests pass on Python 2 and 3. Tested with a customer who was running into threading issues with SES. We confirmed that this patch fixes it. Reviewed at https://reviews.reviewboard.org/r/11727/