Kevin
Could it be that some users have just removed the subject prefix (or their mail clients have not added one?) . I have not used any other mail clients except Eudora or Outlook and these both addd it.
So in order for the threading function to be initiated it checks the first chars of the subject feild for matches on 'RE' ? Then parses an subject index for a match?
Guess its not case sensitive?
BTW, thanks for this really cool mod