The Send Later Thunderbird (and Sea Monkey and Postbox) add-on allows you you to write an email message and then tell Thunderbird when you want it to be sent.The message is then saved back into your Drafts folder, and delivered at approximately the specified time. Release notes for each new release of Send Later are published there and below. Everything below (except for the installation section) assumes that you’ve already installed the add-on from addons.and restarted your mail app.
The default, once per minute, is adequate for most people.
Therefore, when the Send Later dialog pops up, you will only see one of these buttons or the other, as follows: have the “‘Send’ does ‘Send Later'” preference enabled, and you open the dialog with Ctrl-Shift-Enter etc.