If we have used to be emailed feature in the purchase order the following functionality takes place in the purchase order.
If we have created a purchase order without specifying an employee and then we tick the to be emailed checkbox and save the created purchase order, then email is directly sent to the vendor.
If we have assigned a supervisor for an employee and select that employee at the time of purchase order creation then tick the to be emailed check box and save the purchase order. Frist the mail send to the supervisor and when the supervisor clicks the approve button then only an email is sent to the vendor.
If we have assigned a purchase approver for an employee and select that employee and create a purchase order then an email is first sent to the purchase approver when the purchase approver approves the purchase order then the vendor gets the email.