A mail is a message and it is processed based on the output message customizing.
This message is already determined and created when the PO is saved the first time, long before a final release is entered.
The final release will just allow to issue this message.
The only way that I can think of to restrict sending the message based on a value would be to add an own check into a requirement in the messaging schema.
Requirement Routines - Output Control - ABAP Development - SCN Wiki