Configure Thunderbird to send messages at a later time
When you’ve entered a date/time that the add-on understands, the button below it will become active and “Enter a valid date above” will be replaced with what you entered. Click the button or type Ctrl-Enter, or just Enter if your cursor is in the text box, to schedule the message.
You can schedule a one-shot or recurring message using one of the built-in scheduling functions or a function you’ve written or imported yourself.
Send the message using one of the preset buttons. Click “15 mins later”, “30 mins later”, or “2 hours later” to send the message the indicated amount of time into the future.
You can also specify time-of-day and day-of-week restrictions for when your message will be delivered. These restrictions have different (but, I hope, somewhat intuitive) effects based on what kind of scheduling you are doing:
You can save whatever settings you enter as the default settings for when you bring up this dialog in the future, or clear previously saved defaults, by making the appropriate selection above the big scheduled send button, before you click it.