by Jonathan Kamens (Developer) on Aug. 11, 2023 · link permanent

As Scotty once said to Captain Kirk, I can't change the laws of physics. The code for Send Later runs in Thunderbird and therefore Thunderbird needs to be online for scheduled messages to be sent.
Given that there are around 100,000 users of the add-on, it seems that there are a large number of people who don't believe it's "useless."
The user guide offers alternatives if this requirement is not workable for you: https://extended-thunder.github.io/send-later/#thunderbird-must-be-running-for-scheduled-messages-to-be-sent
Giving the add-on a one-star review because it doesn't do something which is literally impossible for it to do is a bit much.