Christian Schulte wrote in
 <7f2954e9-9cef-40bc-9276-4dc236201961@gmail.com>:
 |On 29.08.24 08:56, Walter Alejandro Iglesias wrote:
 |> On Thu, Aug 29, 2024 at 12:35:50AM +0200, Christian Schulte wrote:
 ...
 |Then there is
 |
 |-a file
 |    Attach the given file to the message.[2]
 |
 |and
 |
 |-a
 |    Specify additional header fields on the command line...[3]
 |
 |Good luck.

Yeah it is a mess that caused trouble.  Fwiw my BSD clone uses

  -C "field: body", --custom-header=..

as one approach to define session-lasting custom headers.

 |[1] <https://cvsweb.openbsd.org/src/usr.sbin/smtpd/util.c>
 |[2] <https://linux.die.net/man/1/mail>
 |[3] <https://manpages.debian.org/stretch/bsd-mailx/mail.1.en.html>
 |
 |-- 
 |Christian
 --End of <7f2954e9-9cef-40bc-9276-4dc236201961@gmail.com>

--steffen
|
|Der Kragenbaer,                The moon bear,
|der holt sich munter           he cheerfully and one by one
|einen nach dem anderen runter  wa.ks himself off
|(By Robert Gernhardt)