Setting the default email format on email replies
Although Microsoft Outlook allows you to select a default email format when sending new emails, this setting does not apply when replying to email messages. By default, when composing a reply, Outlook will use the email format of the original email. Thus, no matter if your default format for new mail is set to "plain text", if you are replying to a HTML email, your own reply will be generated as a HTML email.
To increase your Outlook productivity, Bells & Whistles makes it very easy to specify a default email format even when sending replies. As a result, the selected format will be automatically applied on all your email replies, no matter the format of the original email.
To use this feature, you have to enable the option "Always reply using..." from the "Reply Options" section of the Bells & Whistles configuration screen, which can be launched from the Outlook -> Tools menu.
Bells & Whistles for Outlook is shareware ($29.95), try before you buy.