Earlier quoted context omitted.
Usually when I talk to people about the “confusing threads” in email, it turns out that they are using a broken client like Gmail. Threading in email is great. Your email client is broken and not respecting the protocol: https://lwn.net/Articles/837960/
> not respecting the protocol Though optional, every message SHOULD have a "Message-ID:" field. Furthermore, reply messages SHOULD have "In-Reply-To:" and "References:" fields as appropriate, as described below. https://datatracker.ietf.org/doc/html/rfc2822#section-3.6.4 For the uninitiated, SHOULD denotes the following in RFCs: This word, or the adjective "RECOMMENDED", mean that there may exist valid reasons in par…
Most MTAs, in my experience, will add a Message-ID header to a message even if the client did not add one to the original message it sent to the MTA.