References

The References header field may be found in a response and specifies the message IDs of the emails in the thread. (Should be added to with each subsequent reply by appending the Message-ID of the message to which the reply is being made.) If there are more than 12, the first one and then the final 11 are copied, before adding the message ID of the incoming message.

Example:

References: <000801c6c921$aaab3ad0$0c01a8c0@KSBS.local>    <6.2.1.2.0.20060826100953.05b72150@mail.chilkatsoft.com>