Rated 1 out of 5 stars
Unlike the great Stationery this is a PAID (non free) addon. It really should be clearly written in the description.This review is for a previous version of the add-on (2.5.2).
it is - please see About this Add-on under the third heading "licensing". If you want to keep using Stationery, there is no problem as long as you stay behind using Thunderbird 60. I decided to fully monetize after it took several weeks of work to integrate the external HTML files and another several week to make my Add-on compatible with Thunderbird 68.
I also work on my Add-ons every day, providing free support and constantly improving them - had to already do a lot of unpaid work for v2.0 when Thunderbird 60 made a load of Add-ons incompatible. See: https://addons.thunderbird.net/en-US/thunderbird/addon/smarttemplate4/versions/
Also there are many many additional functions for extracting data from the replied to email in the Pro version. The Standard license is currently below 5$ for a whole year which is more than reasonable (renewals are cheaper).
SmartTemplate⁴ is "Free software" in the sense of that you can see all the source code and modify it to suit to your needs - only thing you can't do is create / publish a new version to compete with it. Free as in freedom, not beer.
Unlike a lot of other "free" software, SmartTemplate will never mine your data or sell your information for profit. If you prefer this way of monetization, feel free to use gmail, which reads your mails and tailors advertisements to your needs. I am only a single programmer but I find this behavior highly questionable.