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?
Manually setting in-reply-to mail header with gmail
gmail
I've found a way to do it with Mozilla Thunderbird. Go toEdit -> PreferencesTab Advanced  Subtab General    Button Config EditorThen for the setting mail.compose.other.header, add In-Reply-To. Then you can choose that header when you compose a message just the way you can choose CC/To and others. 