ALTER TABLE `cscart_products` CHANGE `out_of_stock_actions` `out_of_stock_actions` CHAR( 1 ) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL DEFAULT 'S';
I applied the changes offered by Vivek Gupta and it has worked (as always remember to clear the cache) without the need to update the database.
Did some tests and the email is sent when an item goes back in stock.
So is the change to the database only applicable when products are import by CSV?
Actually the Out of stock actions field exists in the list of imported parameters. If you do not use it, the default value will be used. In this case solution from post #3 is required
Note: to update all current products please use the following query:
UPDATE `cscart_products` SET `out_of_stock_actions` = 'S';