Tim Chase wrote in
 <ZbsDtBZVLZQAu3LS@thechases.com>:
 |I noticed that mail(1) doesn't seem to support adding In-Reply-To:
 |headers which ended up messing up some mail workflows here.  Would
 ...
 |I'm not quite certain whether those
 |
 |  head.h_inreplyto = hfield("message-id", mp);
 |
 |lines should be
 |
 |  head.h_inreplyto = skin(hfield("message-id", mp));

It should definetely be skin()ned.

 |The FreeBSD mail(1) source uses the skin() version, but the comment
 |on skin() in the OpenBSD source-tree talks about addresses, not
 |message-IDs.

The BSD Mail codebase is older than the standard.
It is a total looser regarding anything such, and works only by
accident.  And not truly exaggerating.

But when writing out the header you need to place this "msg-id" in
standard terms in <angle-brackets>.
You should also manage References:.

  ...

--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)