I wanted to join a mailing list thread on a list to which I subscribed after the thread started. I wanted my reply to follow an existing message in the thread. 

I researched how I could manually set the `In-Reply-To` header, finding [this](http://www.pixelbeat.org/docs/thunderbird-threading.html) which basically suggests:

> 
> it must be added to the list of custom headers by doing:
> 
> 
> 
> ```
> Edit → Preferences → Advanced → General → Config Editor
> 
> ```
> 
> Search for `mail.compose.other.header` and add `In-Reply-To`
> 
> 
> Then `In-Reply-To` will show up as one of the options in the compose window in the same place where you can choose `To:`, `Cc:`, etc.
> 
> 
> 

Well I did that, and the field did appear (after restarting Thunderbird). I entered the message id in the field and sent the messagge. But the header, although present, was blank:

```
To: "he...
In-Reply-To: 
From: ....
Subject: Re: ...

```

**Is it possible to manually set `In-Reply-To` in a Thunderbird message ?**