It should be on the Standard toolbar, that's displayed when you create an
email. If the toolbar itself is missing, switch it on; if the toolbar is
displayed but the button is missing, reset the toolbar. In both cases,
create a new email and right-clicking any toolbar. If there's no tick in
front of 'Standard' then click it. If the standard toolbar is displayed with
the Send button missing, right-click the toolbar and select Reset.