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Subject

property Subject: string read write;

Example​

procedure SetBusinessSubject(const DocumentNumber: String);
var
  Msg: TIdMessage;
begin
  Msg := Email;
  if Msg = nil then Exit;

  Msg.Subject := 'Invoice ' + DocumentNumber + ' ready for review';
end;

Usage​

Stores decoded subject text for normal MIME-aware header generation while remaining separate from raw Headers in no-encode mode.

Additional Technical Info​

Subject is the typed human-readable topic used for the normal generated Subject header. It is metadata, not body content, a routing guarantee or a trustworthy document identifier.

The property is a direct String field. Assignment does not immediately alter Headers or LastGeneratedHeaders. GenerateHeader later passes Subject through Indy's MIME header encoder using the message's selected header encoding and charset. Non-ASCII text can become one or more RFC 2047 encoded-words, while ordinary safe ASCII can remain readable.

ProcessHeaders performs the reverse operation with DecodeHeader, so normally loaded Subject values are decoded display text rather than encoded-word syntax. This is intentionally different from Organization, whose incoming value is copied raw.

State and output modes​

ClearHeader and Clear reset Subject to empty. Normal generation replaces or removes the generated field according to the current property. ExtraHeaders is appended after typed generation and permits duplicate Subject fields, so do not specify Subject through both paths.

NoEncode bypasses GenerateHeader. In that mode the serialized Subject comes from raw Headers, and changing this property has no effect. Conversely, editing raw Headers and then using normal generation can be overwritten by the typed property.

The setter has no explicit length or control-character validation. Validate external values, remove CR/LF, and apply a sensible operational limit before assignment. Subjects are widely logged, displayed in notifications and exposed to recipients; do not include secrets. They are attacker-controlled on incoming mail, so never authorize a process or select a file solely from Subject text.

Indy does not automatically add or remove Re:. Reply composition owns that policy, and repeated prefixes can reduce threading/display quality even though actual threading should use InReplyTo and References.

The example is source-reviewed only; no message was generated.

External references

Created 2026-07-15