Rated 3 out of 5 stars

The addon only wraps the lines visually. The actual outgoing emails will still have very long lines, which is not good for plain text mailing lists.

This review is for a previous version of the add-on (1.4.1). 

Did you "make sure that "mailnews.send_plaintext_flowed" is disabled" (see add-on description)? If so, please share more details about your setup.
Your use case is also my use case and the primary reason why I'm maintaining this.