RFC 822 allows people to define and use additional header fields in their emails. These fields start with X- in their names. For example, X-Quarantine-ID or X-Spam-Score. Usually these headers are not too interesting.

X-Account-Key

X-DSNContext

X-Mailer

X-MimeOLE

X-Mozilla-Status

X-Mozilla-Status2

X-MS-TNEF-Correlator

X-Original-To

X-Quarantine-ID

X-Spam-Score

X-Spam-Status