One thing that has been annoying me over the past years is that on mailinglists people with Outlook or Outlook Express clients tend to start a new thread by replying to an email in another thread. They remove the body, perhaps some cc: information, change the subject, write their body and send it off. Since Outlook supports completion for to: and cc: fields, it seems a bit of a timewaster to start a new topic like that.
But leaving all that aside, the worst part is that Outlook doesn’t show you all headers, in particular it leaves the
References header intact, which means it is now a reply to an earlier post in another thread. And so threading is broken. You might think “why be annoyed over it”, well, the problem is that online mailinglist indexes use this information for proper navigating through a thread. Typically they provide a ‘next by thread’ and ‘previous by thread’ hyperlink to navigate, but the logical flow of the thread is now broken.
So whenever I see someone with Outlook do this, I send them a note about this in private and most often they adjust the way they work, since, like I stated earlier, starting a new message is actually even faster.