Niklas Hambüchen [off-list ref] writes:
For example, if you pass "--in-reply-to=<msgid>\nTo: <other@example.com"
(notice lack of trailing `>`), then the generated email will actually
contain a
To: [off-list ref]
header.
While I do not think of a reason to specify such a string to the
in-reply-to option (I'd rather edit the output in the editor if I
wanted to do anything fancy [*1*]), I do not think there is a reason
why you want to add a code to forbid such use, either.
[Footnote]
*1* For that matter, --in-reply-to option itself is superfluous.