See https://meta.discourse.org/t/discourse-email-messages-are-incorrectly-threaded/233499
for thorough reasoning.
This commit changes how we generate Message-IDs and do email
threading for emails sent from Discourse. The main changes are
as follows:
* Introduce an outbound_message_id column on Post that
is either a) filled with a Discourse-generated Message-ID
the first time that post is used for an outbound email
or b) filled with an original Message-ID from an external
mail client or service if the post was created from an
incoming email.
* Change Discourse-generated Message-IDs to be more consistent
and static, in the format `discourse/post/:post_id@:host`
* Do not send References or In-Reply-To headers for emails sent
for the OP of topics.
* Make sure that In-Reply-To is filled with either a) the OP's
Message-ID if the post is not a direct reply or b) the parent
post's Message-ID
* Make sure that In-Reply-To has all referenced post's Message-IDs
* Make sure that References is filled with a chain of Message-IDs
from the OP down to the parent post of the new post.
We also are keeping X-Discourse-Post-Id and X-Discourse-Topic-Id,
headers that we previously removed, for easier visual debugging
of outbound emails.
Finally, we backfill the `outbound_message_id` for posts that have
a linked `IncomingEmail` record, using the `message_id` of that record.
We do not need to do that for posts that don't have an incoming email
since they are backfilled at runtime if `outbound_message_id` is missing.
When creating a direct message to a group with group SMTP
set up, and adding another person to that message in the OP,
we send an email to the second person in the OP via the group_smtp
job. This in turn creates an IncomingEmail record to guard against
IMAP double sync.
The issue with this was that this IncomingEmail (which is essentialy
a placeholder/dummy one) was having its Message-ID used as the canonical
References Message-ID for subsequent emails sent out to user_private_message
recipients (such as members of the group), causing threading issues in
the mail client. The canonical <topic/ID@HOST> format should be used
instead for these cases.
This commit fixes the issue by only using the IncomingEmail for the
OP's Message-ID if the OP was created via our handle_mail email receiver
pipeline. It does not make sense to use it in other cases.
Since 3b13f1146b the email threading
in mail clients has been broken, because the random suffix meant
that the References header would always be different for non-group
SMTP email notifications sent out.
This commit fixes the issue by always using the "canonical" topic
reference ID inside the References header in the format:
topic/TOPIC_ID@HOST
Which was the old format. We also add the References header to
notifications sent for the first post arriving, so the threading
works for subsequent emails. The Message-ID header is still random
as per the previous change.
Currently the Message-IDs we send out for outbound email
are not unique; for a post they look like:
topic/TOPIC_ID/POST_ID@HOST
And for a topic they look like:
topic/TOPIC_ID@HOST
This commit changes the outbound Message-IDs to also have
a random suffix before the host, so the new format is
like this:
topic/TOPIC_ID/POST_ID.RANDOM_SUFFIX@HOST
Or:
topic/TOPIC_ID.RANDOM_SUFFIX@HOST
This should help with email deliverability. This change
is backwards-compatible, the old Message-ID format will
still be recognized in the mail receiver flow, so people
will still be able to reply using Message-IDs, In-Reply-To,
and References headers that have already been sent.
This commit also refactors Message-ID related logic
to a central location, and adds judicious amounts of
tests and documentation.
