Danil Smirnov writes:
As per my small investigation, a subscriber Robert N. Evans seems to have "In-Reply-To" headers stripped from the messages that probably causes the thread to break.
I wonder if Hyperkitty is able to leverage some other method to combine the thread correctly in this case?
It's simply not possible to guarantee correct threading if neither References nor In-Reply-To are present.
It is possible to place a message in an approximately appropriate place by threading the threadable messages, grouping that message with threads with "the same" subject, and inserting it (and any descendants) after some message with an earlier date, but this is inherently ambiguous as that message could be a reply to *any* such message with an earlier date.
This would work well if there is a single linear thread. But it is unlikely to work at all well if several posters replied to a single message in the recent past so that there are multiple subthreads active at a given time.
Gmail has a big advantage, since they're reading your mail, indexing it, and creating a fine-grained statistical profile. That database can probably be leveraged for better threading. Or if your posters consistently top-post, it's probably not too hard to match quoted content against the top-level content of an earlier post -- if you have both the development and the computational resources of Google. (Come to think of it, for Gmail this would probably allow them to compress their storage by 50%.) Or maybe they just got lucky.
Steve