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ClearHeader

procedure ClearHeader;

Example​

procedure ReplaceHeadersOnly(const Message: TIdMessage);
begin
  Message.ClearHeader;
  Message.Subject := 'Replacement message';
end;

Usage​

ClearHeader clears address/header/output state and restores selected defaults while deliberately retaining body content and several configuration or state fields.

Additional Technical Info​

ClearHeader resets the header-facing model but leaves Body and MessageParts intact. It performs the following native operations in order:

  • clears CcList, BccList, FromList, NewsGroups, ReplyTo and Recipients;
  • sets Date to zero (UseNowForDate=True), Organization/References/Subject to empty, Priority to mpNormal, and clears receipt-recipient/sender text;
  • assigns empty strings directly to the ContentType/CharSet fields and clears ContentTransferEncoding/ContentDisposition;
  • clears raw Headers, registered-but-ungenerated ExtraHeaders, MIMEBoundary, Flags, MsgId, UID and registered-but-ungenerated LastGeneratedHeaders;
  • assigns meDefault directly to the Encoding field, sets ConvertPreamble true, and resets internal saving/single-part-MIME flags.

Direct field assignments matter. Emptying ContentType here does not call its setter, so it remains truly empty instead of becoming text/plain; charset=us-ascii. Resetting Encoding does not call its setter, so the existing AttachmentEncoding selection is retained.

Several fields are deliberately or accidentally not reset: InReplyTo, IsEncoded, NoEncode, NoDecode, AttachmentEncoding, AttachmentTempDirectory and native events/configuration. A reused message can therefore retain reply threading, raw/encoded-state flags or operational settings. Explicitly reset them when starting a logically unrelated message.

Clearing the MIME boundary does not alter existing parts. Conversely, keeping Body/parts while clearing ContentType/Encoding means GenerateHeader will infer new layout state on the next output pass.

The many sequential mutations are non-transactional and not thread-safe. The source-reviewed example intentionally preserves the existing body/parts and was not runtime-tested.

External references

Created 2026-07-15