Hello mohan,
This is possible with user-exit QQMA0014. Here the code first compares the previous and present statuses and if NOPO is found now which is not the previous status, then a mail will be triggered to the concerned persons through a suitable function module. Concerned persons list will be from a Ztable, which can be like Planner Group vs Email-Id or MainWrkCntr vs EmailID. I hope you understand the logic. Now it is your ABAPer's turn to make it happen. This is quite possible.
After achieving this, when the Notification systatus is changed to NOPO and saved, the mails will be triggered to the specified persons of the PlGrp or WrkCntr of the Notification. (I need not specially mention that the contents of the mail will be accordingly about postponing of the Notification and the number of Notification etc.)
FMs for sending emails
SO_NEW_DOCUMENT_ATT_SEND_API1 - (Send new document with attachments)
SO_NEW_DOCUMENT_ATT_SEND_API1 - (Send new document with out attachment)
Good luck
KJogeswaraRao