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 that 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.
RFC 5322 3.6.4
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 that 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.
Id of the message
Id of other messages