I decided to try the Send Later feature in Thunderbird. I am using T140esr and the About box says it'sup-to-date although I do see references in the forum to later versions.
I couldn't get it to work at first, but got there in the end. I would like to suggest to the developers that the buttons labeled "SEND NOW" and "PUT IN OUTBOX" be removed. These confused me. If I had wanted to send the message now or put it in the outbox, I would not have clicked the "Send Later" button in the first place. The "Send at ...", "In 15 mins", etc. buttons should be the only ones presented to the user.
It's a useful feature and I can see myself using it on a regular basis to send daily and weekly logs as attachments.
Regards Peter M