Fantastic! Rated 5 out of 5 stars

It is probably the best extension available. It organizes sent and received e-mails similarly to what is done by Gmail. It also adds a reply box to the message view, allowing a fast to reply e-mails.

It shouldn't be an extension – it should come by default with Thunderbird!

This review is for a previous version of the add-on (2.6.3).