On Tue, Sep 10, 2024 at 12:07:40PM +0200, Walter Alejandro Iglesias wrote:
> ... body, if you add In-Reply-To or References to 'file', they are
> recognized as such as long as they are accompanied by other headers
> among those recognized by opensmtpd...

I guess these are the headers recognized as such by OpenSMTPD:

Lines 66-82 of src/usr.sbin/smtpd/enqueue.c 
-----------------------------------------------
struct {
	char			*word;
	enum headerfields	 type;
} keywords[] = {
	{ "From:",			HDR_FROM },
	{ "To:",			HDR_TO },
	{ "Cc:",			HDR_CC },
	{ "Bcc:",			HDR_BCC },
	{ "Subject:",			HDR_SUBJECT },
	{ "Date:",			HDR_DATE },
	{ "Message-Id:",		HDR_MSGID },
	{ "MIME-Version:",		HDR_MIME_VERSION },
	{ "Content-Type:",		HDR_CONTENT_TYPE },
	{ "Content-Disposition:",	HDR_CONTENT_DISPOSITION },
	{ "Content-Transfer-Encoding:",	HDR_CONTENT_TRANSFER_ENCODING },
	{ "User-Agent:",		HDR_USER_AGENT },
};


Which means if you don't add any of these headers to the file, the other
headers will go to the body part of the message.


-- 
Walter