Header Fields Known to the MTA

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&#x27;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: Archived-at: (added in MS 8.0) Arrival-date: Article-creation-date: Attachment-date: Attachment-encoding: Attachment-import: Attachment-name: Attachment-type: Attachment: Authentication-results: (added in MS 7u3) 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: (added in iMS 6.0) 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: Deliver-by-date: (added in MS 8.0.1.0) 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: DKIM-Signature: (added in MS 7u1) DL-expanded: DL-expansion-history: DL-expansion: DomainKey-Signature: (added in MS 7u1) DSN-gateway: Encoding-info: Encoding: Encrypted: Envelope-to: (added in MS 8.0.2.2) Error-report: Error: (added in iMS 6.0) Errors-to: Expiration-date: Expires: Expiry-date: Exported-to: Failure: (added in iMS 6.0) Fake-sender: Fcc: Final-log-id: Final-MTA: Final-MTS-type: Final-recipient: Final-status: Followup-to: Form-type: Forwarded-Message-ID: (added in MS 8.0.2.3) Forwarded: From: Fruit-of-the-day-warning: Fruit-of-the-day: Full-name: Future-release-request: (added in MS 7.0) Generate-delivery-report: Header-encoding: Hop-count: HTTP-Proxy: HTTP-User-Agent: Importance: In-reply-to: Incomplete-copy: Information-type: iPlanet-SMTP-Warning: (added in iMS 6.0) 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: (added in JES MS 6.2) MCB-options: MCB-type: MDN-gateway: Media-accept-features: (added in iMS 6.0) Message-context: (added in iMS 6.0) Message-discard: Message-encoding: Message-hash: (added in JES MS 6.2) Message-id: Message-sequence: Message-type: MHS-id: MIC-info: MIME-version: MMDF-Warning: MMHS-Acp127-Message-Identifier: (added in MS 8.0.1.0) MMHS-Codress-Message-Indicator: (added in MS 8.0.1.0) MMHS-Copy-Precedence: (added in MS 8.0.1.0) MMHS-Exempted-Address: (added in MS 8.0.1.0) MMHS-Extended-Authorisation-Info: (added in MS 8.0.1.0) MMHS-Handling-Instructions: (added in MS 8.0.1.0) MMHS-Message-Instructions: (added in MS 8.0.1.0) MMHS-Message-Type: (added in MS 8.0.1.0) MMHS-Originator-PLAD: (added in MS 8.0.1.0) MMHS-Originator-Reference: (added in MS 8.0.1.0) MMHS-Other-Recipients-Indicator-CC: (added in MS 8.0.1.0) MMHS-Other-Recipients-Indicator-To: (added in MS 8.0.1.0) MMHS-Primary-Precedence: (added in MS 8.0.1.0) MMHS-Subject-Indicator-Codes: (added in MS 8.0.1.0) MR-Received: MS-TNEF-Correlator: (added in MS 8.0.1.0) Msg-class: MT-Priority: (added in MS 8.0.1.0) Net-attachment: News-software: Newsgroups: Next-attachment: NNTP-posting-date: NNTP-posting-host: Notification-correlator: Obsoletes: Office: Openpgp: (added in MS 8.0.2.3) Organization: Original-content-disposition: Original-content-type: Original-encoded-information-types: Original-encoding-types: Original-envelope-id: Original-From: (added in MS 8.0.1.0) Original-message-id: Original-MTS-type: Original-recipient: Original-recipients: Original-to: (added in MS 8.0) 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: (added in iMS 6.0) 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-SPF: (added in MS 7u4) 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: Require-Recipient-Valid-Since: (added in MS 8.0) 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: (added in JES MS 6.2) Source-info: Spam-test: (added in iMS 6.0) Status: Structure: Subject-submission-identifier: Subject: Summary: Sun-Java-System-SMTP-Warning: (added in MS 7.0) Sun-ONE-SMTP-Warning: (added in iMS 6.0) Supersedes: Telefax: Text-type: Thread-index: Thread-topic: To: Total-copies-to: Total-to: Transport-options: UA-content-id: UID: UIDL: User-Agent: (added in iMS 6.0) Version: Via-host: VMS-FDL: Wanted-X400-conversion: Warning: (added in iMS 6.0) 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: (added in iMS 6.0) 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: (added in JES MS 6.2) X-Envelope-from: X-Envelope-to: X-EPUB-ListID: X-EPUB-MsgID: X-EPUB-SubID: X-Eric-conspiracy: X-Exchange-Antispam-Report-CFA-Test: (added in MS 8.0.2.2) X-Exchange-Antispam-Report-Test: (added in MS 8.0.2.2) X-EXP32-SerialNo: X-Expiredinmiddle: X-Face: X-Favorite-drink: X-Favourite-drink: X-FAX-defaults: X-FAX-number: X-FAX: X-Finfo: X-Forefront-Antispam-Report: (added in MS 8.0.2.2) X-Forwarded-for: (added in JES MS 6.2) X-Gateway-source-info: X-GBUdb-Analysis: (added in MS 8.0.1.0) X-Genie-from: X-Genie-id: X-Gm-Message-State: (added in MS 8.0) X-Google-DKIM-Signature: (added in MS 8.0) X-Hop-count: X-HPDesk-priority: X-Incognito-format: X-Incognito-SN: X-Info: X-Invoice: (added in MS 8.0) X-Issuer-certificate: X-Job: (added in MS 8.0.2.3) 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: (added in iMS 6.0) X-MessageSniffer-Clean: (added in MS 8.0.1.0) X-MessageSniffer-Identifier: (added in MS 8.0.1.0) X-MessageSniffer-License: (added in MS 8.0.1.0) X-MessageSniffer-RulebaseUTC: (added in MS 8.0.1.0) X-MessageSniffer-Rules: (added in MS 8.0.1.0) X-MessageSniffer-Scan-Result: (added in MS 8.0.1.0) X-MessageSniffer-SNF-Group: (added in MS 8.0.1.0) X-MessageSniffer-Spam: (added in MS 8.0.1.0) X-MessageSniffer-Version: (added in MS 8.0.1.0) X-MessageSniffer-White: (added in MS 8.0.1.0) X-MIC-info: X-Microsoft-Exchange-Diagnostics: (added in MS 8.0.2.2) X-MIME-Autoconverted: X-MIME-version: X-MIMEOLE: X-MIMETrack: X-Mms-3GPP-MMS-Version: (added in MS 8.0) X-Mms-Ack-Request: (added in MS 8.0) X-Mms-Delivery-Report: (added in MS 8.0) X-Mms-Expiry: (added in MS 8.0) X-Mms-Forward-Counter: (added in MS 8.0) X-Mms-Message-Class: (added in MS 8.0) X-Mms-Message-ID: (added in MS 8.0) X-Mms-Message-Type: (added in MS 8.0) X-Mms-MM-Status-Code: (added in MS 8.0) X-Mms-Originator-System: (added in MS 8.0) X-Mms-Previously-sent-by: (added in MS 8.0) X-Mms-Previously-sent-date-and-time: (added in MS 8.0) X-Mms-Priority: (added in MS 8.0) X-Mms-Protocol: (added in JES MS 6.2) X-Mms-Read-Reply: (added in MS 8.0) X-Mms-Request-Status-Code: (added in MS 8.0) X-Mms-Sender-Visibility: (added in MS 8.0) X-Mms-Status-text: (added in MS 8.0) X-Mms-Transaction-ID: (added in JES MS 6.2) X-MS-Attachment: X-MS-Embedded-Report: X-MS-Exchange-CrossTenant-OriginalArrivalTime: (added in MS 8.0.2.2) X-MS-Exchange-Organization-Journal-Report: (added in MS 7u4) X-MS-Has-Attach: (added in MS 8.0.1.0) X-MS-Journal-Report: (added in MS 7u4) 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-Original-From: (added in MS 8.0.1.0) X-Original-MessageID: (added in MS 8.0.1.0) X-Original-To: (added in MS 8.0.1.0) X-OriginalArrivalTime: X-Originating-IP: X-OriginatorOrg: (added in MS 8.0.2.2) X-Perlmx-Spam: (added in JES MS 6.2) 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-Proofpoint-Spam-Details: (added in MS 8.0.1.0) X-Proofpoint-Virus-Version: (added in MS 8.0.1.0) 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-RPost-ClientCode: (added in MS 8.0) X-RPost-Convert-CleanMetadata: (added in MS 8.0) X-RPost-Convert-Pdf-Doc: (added in MS 8.0) X-RPost-Convert-Pdf-Password: (added in MS 8.0) X-RPost-Convert-Pdf-Ppt: (added in MS 8.0) X-RPost-Convert-Pdf-Xls: (added in MS 8.0) X-RPost-Convert-Pdf: (added in MS 8.0) X-RPost-Convert-Zip: (added in MS 8.0) X-RPost-Distrib: (added in MS 8.0) X-RPost-Esign-Expiration: (added in MS 8.0) X-RPost-Esign-Sequential: (added in MS 8.0) X-RPost-Esign-Text: (added in MS 8.0) X-RPost-Esign: (added in MS 8.0) X-RPost-Language: (added in MS 8.0) X-RPost-NoAck: (added in MS 8.0) X-RPost-ReceiptCopy: (added in MS 8.0) X-RPost-ReplyRegistered: (added in MS 8.0) X-RPost-Seal-Hash: (added in MS 8.0) X-RPost-Seal: (added in MS 8.0) X-RPost-SecuRmail-AutoPassword: (added in MS 8.0) X-RPost-SecuRmail-Password: (added in MS 8.0) X-RPost-SecuRmail: (added in MS 8.0) X-RPost-Sidenote-Bcc: (added in MS 8.0) X-RPost-Sidenote-Cc: (added in MS 8.0) X-RPost-Sidenote-Text: (added in MS 8.0) X-RPost-Type: (added in MS 8.0) 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-Source-IP: (added in MS 8.0.1.0) X-SPAM-BAYESIAN__TOKEN: (added in JES MS 6.2) X-SPAM-BDY-QUOTED_EMAIL_TEXT: (added in JES MS 6.2) X-Spam-Flag: (added in MS 8.0) X-SPAM-HDR-IN_REP_TO: (added in JES MS 6.2) X-SPAM-HDR-REFERENCES: (added in JES MS 6.2) X-SPAM-HDR-X_ACCEPT_LANG: (added in JES MS 6.2) X-Spam-Level: (added in iMS 6.0) X-SPAM-META-REPLY_WITH_QUOTES: (added in JES MS 6.2) X-Spam-Score: (added in MS 8.0) X-Spam-Status: (added in iMS 6.0) 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: (added in iMS 6.0) X-URL: X-Virus-Scanned: (added in MS 8.0.1.0) 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: