Damien Cassou <damien@cassou.me> writes:

>                    "To" : "rmod@inria.fr",
>                    "Reply-To" : "rmod@inria.fr",
>                    "From" : "seaside@rmod.inria.fr",
>                    "Subject" : "[rmod] [Mm10s] 2015-11-30",
>                    "Date" : "Mon, 30 Nov 2015 07:00:01 +0100"

A quick look at the code suggests this is falling victim to the
"reply-to munging" detection code, which considers a reply-to field
redudant if it duplicates one of the other fields. From the source

    /* Some mailing lists munge the Reply-To header despite it being A Bad
     * Thing, see http://www.unicom.com/pw/reply-to-harmful.html
     *
     * The munging is easy to detect, because it results in a
     * redundant reply-to header, (with an address that already exists
     * in either To or Cc). So in this case, we ignore the Reply-To
     * field and use the From header. This ensures the original sender
     * will get the reply even if not subscribed to the list. Note
     * that the address in the Reply-To header will always appear in
     * the reply.
     */