Developer reply by Jonathan Kamens
Rated 5 out of 5 stars
UPDATE: The beta version is working as a charm also for 78.4.0 too. You have to setup some options in the add-ons.
--------------
Not working anymore with Thunderbird 78.4.0
It was very helpful for me. Worked on previous versions.
There is a beta release for TB78. Read the top of https://blog.kamens.us/send-later .
To create your own collections, you must have a Mozilla Add-ons account.