[mlmmj] Auto-Reply / Auto-Response?

Hi,

We have an unmoderated mlmmj mailing list here where anyone can
write to. And would like to send an automatic reply to anyone
sending an email to it (something like a "a response can take some
time, please stand-by" text).

So far we tried just creating a file in $listdir/text/ with
"finish-post", "finish-post-request" and
"wait-post-modnonsubposts" but those do not seem to work.

Does an auto-reply feature exist with mlmmj? Ideally the
non-subscribed sender would also only get an auto-reply for the
first message in a newly created mail thread (so basically for any
message without an "In-Reply-To" field).

Regards, Linus