I thought I could use:
mail -s "Subject_here" -a "In-Reply-To: Message_ID_here" < body
but unfortunately the -a is unrecognised.
I found this way of using sendmail instead:
/usr/lib/sendmail -t < body
The file body should have each header on a separate line, then a blank line, then your message body text:
Subject: Example of conversation threading
In-reply-to: <put Message-ID of previous mail here>
Body text here
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2 years ago