Oceniono na 2 z 5 możliwych gwiazdek

Very nice and thoughtfully made add-on, but unfortunately it’s heavily limited: scheduled emails are only sent if Thunderbird is running and the computer is on. This makes the extension practically useless for my purposes, and I think this important limitation should be clearly highlighted.

It's documented in the user guide. There's a whole section there about alternatives if you need something for sending scheduled email when you're offline.
It is not appropriate to give a 2-star review because an extension doesn't do something it isn't designed to do.