From: DeskNow Knowledge Base
http://www.desknow.com/kb/

When viewing a mail thread, only the current mail is shown
http://www.desknow.com/kb/idx/4/098/article/

Mail threading is based on a standard header that most mail clients add to reply messages: "in-Reply-To".

Since the 2.7 release DeskNow tracks mail threads using this techinque.

If you do not see a full thread as expectd, the reason may be one of the following:

1. the sender did not use the 'Reply' button to write the new email. He/she may have written a totally new mail, and just copied part of the original

2. the sender used a mail client that does not add the standard "In-Reply-To" mail header. Most mail clients do, but some still don't.

3. the email was received/sent with a version of DeskNow prior to 2.7