Outlook does not control the format of emails senders chooses to send with.
So, for example, if I write you an email in plain text format, it will NOT convert them to html when you receive them. It will stay as plain text, and will keep it as well when you hit the reply button.
You can even make tests yourself by sending a plain text email.
If you are stating to get plain text emails by people who didn't use to before, either it's because they have changed their settings in Outlook to have plain text, or simply they are now sending you emails via their blackberries/smart phones, which by default is plain text.
