Puntuado con 3 de 5 estrellas

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.

Esta valoración es de una versión anterior del complemento (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.