Header Fields Known to the MTA

From MsgServerDocWiki

The MTA maintains an internal list of "known" header fields. This list is used to perform and in some cases optimize various operations the MTA performs on message headers. In particular, the MTA's header trimming facilities are built on top of this list and only the specific fields on this list can be individually manipulated using header trimming. (The header addition facility, on the other hand, does not depend on this list in the same way and can be used to add arbitrary headers.)

The header fields currently on the list are:

 
A1-format:
A1-forward:
A1-function:
A1-type:
Accept-Language:
Action:
Address:
Addresses-referred-to:
Alternate-recipient:
App-message-id:
Apparently-to:
Application-name:
Approved-by:
Approved:
Arrival-date:
Article-creation-date:
Attachment-date:
Attachment-encoding:
Attachment-import:
Attachment-name:
Attachment-type:
Attachment:
Authors:
Auto-forwarded:
Auto-submitted:
Autoforwarded:
Autosubmitted:
Bcc:
Beak:
Bilateral-Info:
Btw:
Cc:
Certificate:
Change-history:
Change-id:
Comments:
Complete-subject:
Content-access-control:
Content-alternative:
Content-annotation:
Content-charset:
Content-class:
Content-comments:
Content-compression:
Content-correlator:
Content-creation-date:
Content-creator:
Content-description:
Content-disposition:
Content-encoding:
Content-features:
Content-future-object-size:
Content-id:
Content-identifier:
Content-label:
Content-language:
Content-last-modification-date:
Content-last-modifier:
Content-last-read-date:
Content-last-reader:
Content-legal-qualifications:
Content-length:
Content-lines:
Content-machine:
Content-MD5:
Content-mode:
Content-notification:
Content-object-size:
Content-operating-system:
Content-pathname:
Content-permitted-actions:
Content-privacy:
Content-related-stored-file:
Content-return:
Content-service:
Content-storage-account:
Content-transfer-encoding:
Content-type:
Conversation-id:
Conversion-with-loss:
Conversion:
Copies-to:
Data-description:
Data-encoding:
Data-name:
Data-type:
Date-delivered:
Date-posted:
Date-received:
Date-transferred:
Date-warning:
Date:
Deferred-delivery:
Defragment-failed:
DEK-info:
Delivered-to:
Delivery-date:
Delivery-receipt-to:
Delivery-report-content-billing-information:
Delivery-report-content-intermediate-trace:
Delivery-report-content-original:
Delivery-report-content-reported-recipient-info:
Delivery-report-content-UA-Content-id:
Destination-application:
Diagnostic-code:
Discarded-X400-IPMS-Extensions:
Discarded-X400-MTS-Extensions:
Disclose-recipients:
Disposition-notification-options:
Disposition-notification-to:
Disposition:
Distribution:
DL-expanded:
DL-expansion-history:
DL-expansion:
DSN-gateway:
Encoding-info:
Encoding:
Encrypted:
Error-report:
Error:
Errors-to:
Expiration-date:
Expires:
Expiry-date:
Exported-to:
Failure:
Fake-sender:
Fcc:
Final-log-id:
Final-MTA:
Final-MTS-type:
Final-recipient:
Final-status:
Followup-to:
Form-type:
Forwarded:
From:
Fruit-of-the-day-warning:
Fruit-of-the-day:
Full-name:
Generate-delivery-report:
Header-encoding:
Hop-count:
HTTP-Proxy:
HTTP-User-Agent:
Importance:
In-reply-to:
Incomplete-copy:
Information-type:
iPlanet-SMTP-Warning:
Issuer-certificate:
Key-info:
Keywords:
Language:
Last-attempt-date:
Latest-delivery-time:
Licensed-to:
Lines:
List-Archive:
List-Digest:
List-Help:
List-Host:
List-Id:
List-Info:
List-Owner:
List-Post:
List-Software:
List-Subscribe:
List-Unsubscribe:
Mac-fcreator:
Mac-ftype:
Mail-followup-to:
Mail-system-version:
Mailer:
Mailing-List:
MCB-options:
MCB-type:
MDN-gateway:
Media-accept-features:
Message-context:
Message-discard:
Message-encoding:
Message-hash:
Message-id:
Message-sequence:
Message-type:
MHS-id:
MIC-info:
MIME-version:
MMDF-Warning:
MR-Received:
Msg-class:
Net-attachment:
News-software:
Newsgroups:
Next-attachment:
NNTP-posting-date:
NNTP-posting-host:
Notification-correlator:
Obsoletes:
Office:
Organization:
Original-content-disposition:
Original-content-type:
Original-encoded-information-types:
Original-encoding-types:
Original-envelope-id:
Original-message-id:
Original-MTS-type:
Original-recipient:
Original-recipients:
Originating-client:
Originator-and-DL-expansion-history:
Originator-certificate:
Originator-ID-asymmetric:
Originator-ID-symmetric:
Originator-info:
Originator-return-address:
Originator:
P1-content-type:
P1-message-id:
P1-recipient:
P2-originator:
Path:
Phone:
PMDF-SMTP-Warning:
Posted:
Posting-date:
Posting-status:
PP-Warning:
Precedence:
Prefer-Language:
Preferred-Language:
Prev-Resent-date:
Prev-Resent-from:
Prev-Resent-to:
Prevent-nondelivery-report:
Priority:
Proc-type:
Re-sent-by:
Read-receipt-to:
Received-by:
Received-from-MTA:
Received-from:
Received:
Recipient-ID-asymmetric:
Recipient-ID-symmetric:
Recipient-ID:
Recipient-info:
Recipient-reassignment:
Recipient:
Redirection-history:
References:
Refused-by:
Registered-mail-reply-requested-by:
Rejected-for:
Remote-MTA:
Remote-MTS-type:
Remote-recipient:
Remote-status:
Reply-by:
Reply-copies-to:
Reply-to:
Reporting-DL-name:
Reporting-MTA:
Reporting-UA:
Repository:
Requested-delivery-method:
Resent-bcc:
Resent-cc:
Resent-date:
Resent-from:
Resent-message-id:
Resent-reply-to:
Resent-sender:
Resent-to:
Resent-wide-reply-to:
Respond-by:
Ret-message:
Return-path:
Return-receipt-requested:
Return-receipt-to:
Send-to:
Sender-ID:
Sender:
Sensitivity:
Service-message:
Session-id:
Signature:
SMF-version:
Solicitation:
Source-info:
Spam-test:
Status:
Structure:
Subject-submission-identifier:
Subject:
Summary:
Sun-Java-System-SMTP-Warning:
Sun-ONE-SMTP-Warning:
Supersedes:
Telefax:
Text-type:
Thread-index:
Thread-topic:
To:
Total-copies-to:
Total-to:
Transport-options:
UA-content-id:
UID:
UIDL:
User-Agent:
Version:
Via-host:
VMS-FDL:
Wanted-X400-conversion:
Warning:
Warnings-to:
Wide-Reply-to:
Will-retry-until:
X-10:
X-11:
X-12:
X-1:
X-2:
X-3:
X-4:
X-5:
X-6:
X-7:
X-8:
X-9:
X-Accept-Language:
X-Address:
X-Admin:
X-Advertisement:
X-attachments:
X-Authentication-warning:
X-Authentication:
X-Author-info:
X-BCC:
X-BeenThere:
X-BFT:
X-BTW:
X-Buckslip:
X-CC:
X-Certificate:
X-Charset:
X-CLREnv-To:
X-Comment:
X-Complaints-to:
X-Confirm-reading-to:
X-Content-description:
X-Content-disposition:
X-Content-id:
X-Content-transfer-encoding:
X-Content-type:
X-Corrupt-Content:
X-DEK-info:
X-Die-Spammers:
X-Dispatcher:
X-DMW-Body-names:
X-eGroups-Return:
X-Envelope-from:
X-Envelope-to:
X-EPUB-ListID:
X-EPUB-MsgID:
X-EPUB-SubID:
X-Eric-conspiracy:
X-EXP32-SerialNo:
X-Expiredinmiddle:
X-Face:
X-Favorite-drink:
X-Favourite-drink:
X-FAX-defaults:
X-FAX-number:
X-FAX:
X-Finfo:
X-Forwarded-for:
X-Gateway-source-info:
X-Genie-from:
X-Genie-id:
X-Hop-count:
X-HPDesk-priority:
X-Incognito-format:
X-Incognito-SN:
X-Info:
X-Issuer-certificate:
X-Juno-Line-Breaks:
X-Key-info:
X-Keywords:
X-Licensed-to:
X-List-Archive:
X-List-Digest:
X-List-Help:
X-List-Host:
X-List-Id:
X-List-Info:
X-List-Owner:
X-List-Post:
X-List-Software:
X-List-Subscribe:
X-List-Unsubscribe:
X-List:
X-Listname:
X-Listprocessor-Version:
X-Listserver:
X-Listservers:
X-Loop:
X-Lotus-FromDomain:
X-LSV-ListID:
X-Machine:
X-Mailer:
X-Mailing-List:
X-Mailman-Version:
X-Message-flag:
X-MIC-info:
X-MIME-Autoconverted:
X-MIME-version:
X-MIMEOLE:
X-MIMETrack:
X-Mms-Protocol:
X-Mms-Transaction-ID:
X-MS-Attachment:
X-MS-Embedded-Report:
X-MSMail-conversation-id:
X-MSMail-message-id:
X-MSMail-priority:
X-MSXMTId:
X-MTS-LoopDetect:
X-MTS-Priority:
X-MTS:
X-MyDeja-info:
X-Netmessenger-type:
X-News-software:
X-Newsgroups:
X-Newsreader:
X-No-Archive:
X-Notes-form:
X-Notes-item:
X-NS-transfer-id:
X-NVL-Content-charset:
X-NVL-Content-filename:
X-NVL-Content-modification-date:
X-NVL-Content-transfer-encoding:
X-NVL-Content-type:
X-OldDate:
X-Open-Mail-hops:
X-Orcl-application:
X-Orcl-content-type:
X-Org-addr:
X-Org-misc:
X-Organisation:
X-Organization:
X-Orig-sender:
X-OriginalArrivalTime:
X-Originating-IP:
X-Perlmx-Spam:
X-PGP-signed:
X-PGP-version:
X-Phone-number:
X-Phone:
X-PipeGCOS:
X-Pipehub:
X-Pipeuser:
X-PMrqc:
X-PMuue:
X-Priority:
X-Proc-type:
X-PS-Qualifiers:
X-Received:
X-Recipient-ID:
X-Reply-to:
X-Report-type:
X-Reposting-Policy:
X-Resent-bcc:
X-Resent-cc:
X-Resent-date:
X-Resent-from:
X-Resent-message-id:
X-Resent-to:
X-Save-Headers:
X-Save-Outgoing:
X-Scanner:
X-Security:
X-Sender-ID:
X-Sender-IP:
X-Sender:
X-Sent-Mail:
X-Server-Date:
X-SMAP-Received-from:
X-SMTP-Client:
X-SPAM-BAYESIAN__TOKEN:
X-SPAM-BDY-QUOTED_EMAIL_TEXT:
X-SPAM-HDR-IN_REP_TO:
X-SPAM-HDR-REFERENCES:
X-SPAM-HDR-X_ACCEPT_LANG:
X-Spam-Level:
X-SPAM-META-REPLY_WITH_QUOTES:
X-Spam-Status:
X-Spook:
X-Status:
X-Sun-Charset:
X-Sun-Content-encoding:
X-Sun-Content-label:
X-Sun-Content-length:
X-Sun-Content-lines:
X-Sun-Data-description:
X-Sun-Data-name:
X-Sun-Data-type:
X-Sun-Encoding-info:
X-Sun-Text-type:
X-Sybari-Space:
X-To:
X-Trace:
X-Truth:
X-UID:
X-UIDL:
X-URI:
X-URL:
X-VMS-Cc:
X-VMS-From:
X-VMS-To:
X-WebMail-Urgent:
X-WebMail-UserId:
X-WM-Posted-At:
X-X-Sender:
X-Yow:
X400-Content-correlator:
X400-Content-identifier:
X400-Content-return:
X400-Content-type:
X400-MTS-identifier:
X400-Originator:
X400-Received:
X400-Recipients:
X400-Trace:
Xref:

Note that this list contains a large number of nonstandard header fields in addition to most of the standardized fields.

Note also that additions to the list are made fairly regularly so what appears here may be somewhat out of date.