Sender: This header is unusual in email (X-Sender: is usually used instead), but appears occasionally, especially in copies of Usenet posts. It should identify the sender. In the case of Usenet posts, it is a more reliable identifier than the From: line.

Subject: A completely free-form field specified by the sender, intended, of course, to describe the subject of the message.

To: The "message To: "described above. Note that the To: header need not contain the recipient's address!

Mailing-List: sometimes in mailing lists to identify the name of the actual list - in our case, it's someassociation@yahoogroups.com. They are even nice enough to provide an address for the list owner to aid in abuse issues. They even provide the unsubscribe address!

Delivered-To: Usually for mailing lists; it shows the list's address.

Precedence: Again usually for mailing lists. Not very significant, unless being used to analyze content for junk mail rules.

X-headers is the generic term for headers starting with a capital X and a hyphen. The convention is that X-headers are nonstandard and provided for information only, and that, conversely, any nonstandard informative header should be given a name starting with "X-". This convention is frequently violated.

X-Confirm-Reading-To: This header requests an automated confirmation notice when the message is received or read. It is typically ignored. Presumably, some software acts on it.

X-Distribution: In response to problems with spammers using his software, the author of Pegasus Mail added this header. Any message sent with Pegasus to a sufficiently large number of recipients has a header added that says "X-Distribution: bulk". It is explicitly intended as something for recipients to filter against.

X-Errors-To: Like Errors-To: This header specifies an address for errors to be sent to. It is probably less widely obeyed.

X-Mailer: (also X-mailer:) A freeform header field intended for the mail software used by the sender to identify itself (as advertising or whatever.) Since much junk email is sent with mailers invented for this purpose, this field can provide much useful fodder for filters. Typical of Micro$oft to plug their MIME here.

X-PMFLAGS: This is a header added by Pegasus Mail. Its semantics are not obvious. It appears in any message sent with Pegasus, so it doesn't obviously convey any information to the recipient that isn't covered by the X-Mailer: header.

X-Priority: Another priority field, used notably by Eudora to assign a priority (which appears as a graphical notation on the message.)

X-Sender: The usual email analogue to the Sender: header in Usenet news. This header purportedly identifies the sender with greater reliability than the From: header. In fact, it is nearly as easy to forge, and should therefore be viewed with the same sort of suspicion as the From: header.

X-UIDL: This is a unique identifier used by the POP protocol for retrieving mail from a server. It is normally added between the recipient's mail server and the recipient's actual mail software. If mail arrives at the mail server with an X-UIDL: header, it is probably junk (there's no conceivable use for such a header, but for some unknown reason many spammers add one.)

Congratulations if you got this far! I hope this was as useful as I intended. Credit is due to Ken Lucke who mastered this back in 1997. Thanks Ken!