Purchase order automatically will send to vendor via e-mail after final release, then set Dispatch time as 4 {Send immediately (when saving the application)} in NACE t.code in application EF for your PO output type and save.
Also maintain condition record for PO output type in t.code: MN04 and keep Dispatch time as 4 {Send immediately (when saving the application)} with other parameters (document type, purchase organization etcu2026.)
For more check links for details setting e-mail setting for PO
http://www.architectsap.com/blog/sap/sap-mm-purchasing-send-purchase-order-by-mail-to-vendor-in-sap/
http://www.sap-img.com/materials/external-send-sending-po-by-email.htm