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ConvertPreamble

property ConvertPreamble: Boolean read write;

Example​

procedure PreserveLiteralPreamble;
var
  Msg: TIdMessage;
begin
  Msg := Email;
  if Msg = nil then Exit;

  // Keep Body as the MIME preamble; create explicit TIdText parts for content.
  Msg.ConvertPreamble := False;
end;

Usage​

ConvertPreamble controls whether qualifying multipart output appends a quoted-printable text part copied from Body instead of emitting Body as the MIME preamble.

Additional Technical Info​

ConvertPreamble changes how normal MIME body generation handles a nonempty Body. Its default is True, and ClearHeader—therefore Clear and load—resets it to True.

Conversion occurs only when all of these conditions are true during full body output:

  • message Encoding is meMIME;
  • the generated layout is not single-part MIME;
  • Body contains more than spaces, tabs and CR/LF characters;
  • MessageParts contains at least one item; and
  • its most recently counted TextPartCount is zero.

Indy then appends a TIdText item containing Body, copies the message CharSet, sets ContentType to text/plain and ContentTransfer to quoted-printable, emits a standard MIME preamble, and writes the new last item first.

Important persistent-mutation quirk​

Despite its conversion-helper role, the appended TIdText is not removed after output. It remains owned by MessageParts. The current send uses it once, but a later send/save first recounts the now-present text part and no longer takes the conversion branch. The later output can therefore emit the original Body as preamble and also emit the retained part containing the same text. Repeated serialization is not necessarily idempotent.

To avoid hidden collection mutation, prefer explicit TIdText construction and keep Body empty or intentionally use it as a preamble. If using automatic conversion, inspect or clear the resulting parts before serializing the same object again. ClearBody removes both Body and the appended part.

When the property is False, qualifying Body content is encoded/written as literal preamble; it is not promoted into a MIME entity and has no part headers. With no parts, Body is written directly regardless of this property. NoEncode bypasses this logic entirely.

The example only changes an in-memory field and was source-reviewed, not executed or sent.

External references

Created 2026-07-15