Now, I do not know if there is an RFC documenting In-Reply-To.
Pointers welcome. I've relied on my experience with it since '88/89
when I first starting using email.
"In-Reply-To" ":" *(phrase / msg-id)
"References" ":" *(phrase / msg-id)
That is, the In-Reply-To and References field contain zero or more occurances
of a "phrase" field (plain text) or a Message ID.
The contents of this field identify previous correspon-
dence which this message answers. Note that if message iden-
tifiers are used in this field, they must use the msg-id
The contents of this field identify other correspondence
which this message references. Note that if message identif-
iers are used, they must use the msg-id specification format.