If you want to send an order acknowledgement via email, go first into the order's detailed view and scroll down to the bottom. Select Email as Action and click on Save.
Note that you must have a recipient adress specified on the customer in the customer register under Document Delivery Options in order to be able to send an order via email.
You must also have a sender address registered under Settings - Sales Order/Invoicing/Purchase Order - Email documents.
An email with the order acknowledgement as an attachment is sent to the customer. In the list, there is a symbol indicating that the order was sent by email. If you hold the cursor over the icon you see when the mailing occurred as well as to which recipient.