You may be interested in how to get plain text emails using Thunderbird with universities that use Office 365. FIrst off, you need to use Owl for Exchange (paid add-on).

• Main OWL account: change the main setting for composing to plain text under “account settings” and then “composition & addressing”. The option in the general Thunderbird settings seems to do nothing.
• Edit the ofllowing three variables in the “config editor”

plain_text.wrap_long_lines false
mail.wrap_long_lines true 
mailnews.wraplength  0