Hello Harry,
It may be sent out through a Message Type, However, when the settings are not favorable to create a output type in PO, it should not even appear in the PO messages while creating the PO.
The case which you are explaining with message type may suitable for Master Data, not for Transactional Data like PO, Delivery, etc as the output triggers through output types.
Let me know if my understanding is wrong.
With Regards,
Vishnu