In the purchase order edit screen, click on Save and send button.
After clicking on the "Save and send" button, a send window will pop up. You can proceed to,
Write an e-mail message to your vendor, and click "Send".
Click "Later" to keep this PO as draft
Click "Skip" to mark this PO as "Sent".
Now you will be re-directed back to the PO list screen, where you can find the newly sent PO marked as "Sent".
You can "View/Edit", "Convert to bill", or "Send again" in its "Action" or use the filters to find other purchase orders in a certain period and/or with certain number/vendor.