Rated 2 out of 5 stars

Why requiring a "+" in the recipient address ?? Almost nobody has a recipient address with a "+".

This review is for a previous version of the add-on (1.0). 

Hi, this addon was made to fulfill the wishes of a single user I saw on IRC. GMail (and probably other services) route emails sent to e.g. user123+foobar@gmail.com to user123@gmail.com.
GMail doesn't allow you to send emails as user123+foobar@gmail.com, but some services do. This extension is for people using services that do.
What's your use case? Feel free to checkout out https://blog.qiqitori.com/?p=194 or https://github.com/qiqitori/reply_as_original_recipient to see if any other branches (1.1, 1.2) address your needs.