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From: Alex Musolino <alex@musolino.id.au>
Date: Mon, 17 Feb 2020 10:24:41 +1030
To: 9front@9front.org
Subject: Re: [9front] [PATCH] upas/Mail: add In-Reply-To to replies
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> +		if(strlen(m->messageid) > 0)
> +			Bprint(r->w->body, "In-Reply-To: %s\n", m->messageid);

You need to enclose the messageid with angle brackets, i.e.

	Bprint(r->w->body, "In-Reply-To: <%s>\n", m->messageid);

--
Cheers,
Alex Musolino