Hi,
As I mentioned we're going to look into it. We are most likely not going to inline the entire previous conversation in replies, but at least preserve the original subject.
Having said that though, "threading" is something that is done by the e-mail client and not inside the e-mail itself. Thunderbird, Outlook and GMail for instance can tell which e-mails belong together by a "cross reference id" system and should present the initial reply from our system paired with the original you sent without any problem. The habit of including the entire e-mail one is replying to is not an absolute, although a common habit, introduced by Microsoft Outlook.
But, as I said, we'll look into it.