No. You may talk about these headers:
In-Reply-To: <topic/16501@meta.discourse.org>
References: <topic/16501@meta.discourse.org>
It has been explained here why it is wrong:
peternlewis:
References: lists the parent’s References header followed by the parent’s message ID.
If you look at ours currently, it looks like this:
Message-ID: <539a986f8300_5a3fae28acd8c8201e9@forum.mail>
and
Message-ID: <539a9904d40bd_5a3fae28d5b8242036d@forum.mail>
which is clearly wrong. The first one was the original PM, so should not have an In-Reply-To or References header, and the second one (the reply, should look like this:
Message-ID: <539a9904d40bd_5a3fae28d5b8242036d@forum.mail>
And a reply to that reply should look like this:
Message-ID: <adsassadfdfsdfs_sddfsdfsaererfgdsf@forum.mail>
In-Reply-To and References must point to a unique message id. <topic/16501@meta.discourse.org> is not a unique message id thus mail clients cannot do threading. It works when I replace these headers with its correct message ids manually on the imap server.
edit:
Message-ID: <discourse/onebox/pull/257/r20941543@github.com>
In-Reply-To: <discourse/onebox/pull/257@github.com>
References: <discourse/onebox/pull/257@github.com>
Maybe fixing the Message-ID in discourse mails suffices. Dicourse does it this way (wrong!):
Message-ID: <5475a885c34fd_2fd5036a24480ae@archvm.mail>
In-Reply-To: <topic/753@discourse.sevenbyte.org>
References: <topic/753@discourse.sevenbyte.org>