Yes, the built-in Send Later requires manual input, but the Send Later add-on offers automatic, scheduled sending:

https://addons.thunderbird.net/en-US/thunderbird/addon/send-later-3/

https://support.mozilla.org/en-US/kb/installing-addon-thunderbird

Thanks - is there someway of seeing what emails you have queued to send later with this? Seems there is also an option for a Send Later button - I've installed both, but don't see it. Also, how can you see what Add-ons yu've added on already?

According to the user guide, scheduled messages are stored in the Drafts folder. The guide also explains how to add a button.

Tools/Add-ons/Extensions lists all installed add-ons of this type.

Thanks sfhowes,

I've managed to add the button - I was trying to add it to the main toolbar, rather than the one in Write!

It also never occurred to me that Extensions was the same as Add-ons!

OK, I'm stupid!

sfhowes said

According to the user guide, scheduled messages are stored in the Drafts folder. The guide also explains how to add a button. Tools/Add-ons/Extensions lists all installed add-ons of this type.

Thanks, that helped. I found that "send later" messages weren't stored in my "Drafts" folder, but in my "Local Folders" > "Outbox" (thunderbird 68.2.1)

/charles