SI会社なのに意味なくリプライでメールを送ってるひとが多くて気持ち悪いので、それを指摘するためにRFC2822を読んで、Message-ID と References の関係の裏をとってみた。

Though optional, every message SHOULD have a "Message-ID:" field.Furthermore, reply messages SHOULD have "In-Reply-To:" and "References:" fields as appropriate, as described below.

The "In-Reply-To:" and "References:" fields are used when creating a reply to a message. They hold the message identifier of the original message and the message identifiers of other messages (for example, in the case of a reply to a message which was itself a reply). The "In-Reply-To:" field may be used to identify the message (or messages) to which the new message is a reply, while the "References:" field may be used to identify a "thread" of conversation.