Avaliado com 3 de 5 estrelas

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 análise é para uma versão anterior do extra (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.