Noone has no idea of to whom you were writing to - and why - above.
You cannot verify this. And in fact what you is factually incorrect, at
least in my case.
His email contains the In-Reply-To header, that allows Thunderbird to
correctly put his message in its place in the thread.
I infer that you may be using an email client that does not use the
In-Reply-To header to thread messages, but only the subject, and is
therefore fooled by subject changes, which are useful in many situations.
If my supposition is correct, you may want to try a better email client.