When a message comes from the list, there is a header with a message ID in it. The header is called Message-ID.
When you reply, your mail client sends the message specifying that it is in response to that Message-ID.
The converse applies to us digest subscribers. The whole digest comes with the subject, "fedora-list Digest, Vol n, Issue m". All replies have to be new messages; there is no way to reply to a message within an individual thread with "reply" in the email client unless you hijack the digest topic itself.
There's nothing to do for email clients that follow the "Message-ID" in that instance, I suppose.
