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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?

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up vote 3 down vote accepted

I've found a way to do it with Mozilla Thunderbird.

Go to

 -> Preferences

Tab Advanced
  Subtab General
    Button "Config Editor"

Then 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.

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