我发现用于解码50MB的MIME消息(简单的文本
附件),TMimeMess使用了600-800MB的内存.

解码码在这里：

FMimeMsg.Header.DecodeHeaders(FMimeMsg.MessagePart.Lines);
FMimeMsg.MessagePart.DecomposeParts;
ExtractPartsFromMess(FMimeMsg.MessagePart);
...
procedure ExtractPartsFromMess(AMimePart: TMimePart);
var i: integer;
begin
  if AMimePart.GetSubPartCount = 0 then
    Begin
      if (AMimePart.PrimaryCode = MP_TEXT) then
        Begin
          AMimePart.DecodePart;          
          FBody.Add(string(ReadStrFromStream(AMimePart.DecodedLines,AMimePart.DecodedLines.Size)));
    end
  else if (AMimePart.PrimaryCode = MP_BINARY) then
    Begin
      if AMimePart.FileName <> '' then
        Begin
          AMimePart.DecodePart;
          AMimePart.DecodedLines.SaveToFile(FPathToIncomingFolder+AMimePart.FileName);
        end;
    end;
end
else
Begin
  for i:=0 to AMimePart.GetSubPartCount-1 do
    Begin
      ExtractPartsFromMess(AMimePart.GetSubPart(i));
    end;
end;
end;

谢谢.