Click on "Invoices" in the sidebar menu.
Adjust the filters as desired to filter out from all available invoices.
Select the invoices that you want to send by ticking the boxes beside the invoice number.
Click on "More Actions" and in the dropdown select "Email".
Edit the list of person to whom the mail is to be sent. You can select contacts from contact master as well as add it manually.
Select the type of template according the type of invoice.
Click on "Send" to send the mail.