[donaldguy]

I mean, depends on your MUA in practice. I think by standard for server side threading— https://tools.ietf.org/html/rfc5256 — "base subject" is definitely a valid consideration. I only skimmed though. :-) feel free to extract a spec, I'm curious (but not enough to read 19 pages right now)

[zAy0LfpBZLC8mAC]

If I may quote:

      ORDEREDSUBJECT

         The ORDEREDSUBJECT threading algorithm is also referred to as
         "poor man's threading".  The searched messages are sorted by

In other words: as a workaround for garbage email clients and services too incompetent to use email correctly.

Yes, the REFERENCES algorithm also uses the base subject--but never to override message-id based threading, only for merging together threads that couldn't be joined using message-ids ... or in other words: as a workaround for garbage email clients and services too incompetent to use email correctly.

Well, and for merging together chunks of a thread where you have major gaps in your mailbox, so that reference headers don't overlap enough to make the connection, but the point remains that it's not really a threading mechanism, just a last-ditch attempt to recover a thread where proper threading information is missing, which is fine for corner cases like large gaps in which messages you have in your mailbox, and doesn't change that a sender that doesn't supply reference headers in the first place is incompetent.