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.
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Not working anymore with Thunderbird 78.4.0
It was very helpful for me. Worked on previous versions.

This review is for a previous version of the add-on (7.4.4). 

There is a beta release for TB78. Read the top of https://blog.kamens.us/send-later .