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