About this Add-on
There are no new/own functions. The button is just a simple way to always have the send later option visible and reachable by just one click.
You can use the "Send later" button with standard Thunderbird to save your new mail in the "Outbox". In case you're using the additional add-on "Send Later" (by jikamens), the button is completely compatible with this add-on and invokes its dialog.
This add-on is compatible with jikamens "Send later" add-on.