How to turn off AutoCorrect for the original message text
To turn off AutoCorrect for the original text of the previous messages, do the following:
1. On the File tab, click Options:

2. In the Outlook Options dialog box, on the Mail tab, under Compose message, check the Ignore original message text in reply or forward check boxes:

3. Click OK.
See also this tip in French: Comment désactiver la correction automatique pour le texte du message d'origine.