This question already has an answer here:
I'm using the gmail web interface, and I've happened to read an online mailing list archive. I want to reply to a specific message of a user, and I already know its message-id.
To make my reply be in the correct thread, I found I need to to set the in-reply-to header to the message-id of that mail I want to reply to.
Is that possible with gmail? Is it possible with any email program that has a modern GUI?