RFC1036 defines the Standard for Interchange of USENET Messages. The headers that are at the top of a USENET message are an important part of this definition. There are six headers that are absolutely required they are:

"From", "Date", "Newsgroups", "Subject", "Message-ID", and "Path".

There are thirteen headers that are standard but optional they are:

"Followup-To", "Expires", "Reply-To", "Sender", "References", "Control", "Distribution", "Keywords", "Summary", "Approved", "Lines", "Xref", and "Organization".

The interesting part is that the RFC also states that “Any unrecognized headers are allowed, and will be passed through unchanged.” This allows users and people who implement news clients to extend the set of headers. An informal convention of placing “X-“ for eXperimental before the non-standard headers evolved. Probably the two best known examples of X-Headers that have come into common use are X-Mailer and X-No-Archive. A check of 1617 articles in he newsgroup news.groups turned up the following list of non-standard USENET headers.

Answer:
Archive-Name:
CC:
Cache-Post-Path:
Cc:
Changes-posted-to:
Comment:
Comments:
Content-Disposition:
Content-Transfer-Encoding:
Content-Type:
Copyright:
Directory:
Email:
In-Reply-To:
In-reply-to:
Last-Modified:
Last-change:
Login:
MIME-Version:
Mail-To-News-Contact:
Mail:
Mark:
Mime-Version:
NNTP-Posting-Host:
NNTP-Posting-User:
Note:
Original-author:
Originator:
Posted-By:
Posting-Frequency:
To:
URL:
Version:
Web:
X-001:
X-002:
X-Access:
X-Address-Invalid:
X-Admin:
X-Advice:
X-Also-Approved:
X-Apparently-To:
X-Approved:
X-Article-Creation-Date:
X-Attribution:
X-Auth:
X-Authenticated-Sender:
X-Authenticated:
X-Bomb-Instructions:
X-Comment:
X-Content-Currency:
X-Copyright:
X-Details:
X-Face:
X-Geek-:
X-Geek-Code:
X-Geek:
X-Given-Sender:
X-HTML:
X-Home-Pages:
X-Homepage:
X-Http-User-Agent:
X-Incantation:
X-Junk-Policy:
X-Last-Updated:
X-Legal-Warning:
X-Mail-Policy:
X-Mail2News-Errors-To:
X-Mail2News-Info-Url:
X-Mail2News-Path:
X-Mail2News-User:
X-Mailer:
X-Marks:
X-Mayan-Date:
X-Meow:
X-NETCOM-Date:
X-NNTP-Poster:
X-NNTP-Posting-Host:
X-Newsposter:
X-Newsreader:
X-Nntp-Posting-Host:
X-Nntp-Posting-User:
X-No-Archive:
X-No-Junk-Mail:
X-Note:
X-Operating-System:
X-Origin:
X-Original-Message-ID:
X-Original-Newsgroups:
X-Original-Path:
X-Originating-IP-Addr:
X-PGP-Ox6E86B769:
X-Perfectly-Legal-To-Mention:
X-Posted-By:
X-Posted-For-Grubor-By:
X-Posted-From:
X-Posting-Agent:
X-Posting-Frequency:
X-Priority:
X-Received-On:
X-Robomod:
X-SCRM-Policy:
X-Sender:
X-Server-Date:
X-Sound-Advice:
X-Spam-Bait:
X-Spam:
X-Spambait:
X-Stanley-Cup-Champion:
X-To:
X-Trace:
X-URL:
X-Voice:
X-XS4ALL-Date:
X-XXDate:
X-XXMessage-ID:
X-easynet-cherry:
X-headers:
X-newsgroup:
X-rn/trn/strn-Killfile-Entry:
X-unsolicited-email:
X-windows:

List courtesy of Simon Lyall (simon@darkmere.gen.nz)

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