About this Add-on
This is a paid addon. See the website https://www.beonex.com/owl/ for prices. The first month is free, so that you can test it and see whether you like it.
The developers of this addon have contributed many parts to Thunderbird as Open Source, and they are active around Thunderbird and its predecessor since many years, in part since almost 20 years. This is an attempt to earn a living.