Email generation after posting a document in FB60

I am working on FB60 transaction. After I enter a vendor invoice, I am now trying to post the document. While i save the document there is a popup which shows the cross company code number, document number, year and company code.

Now at this point when the popup window appears, there must be a mail sent to the respective users with the details seen in popup.

I am able to find the function module for shooting mail but my difficulty is i am not able to trace the point where i should add this FM , however I am debugging this screen to find out.

I would request you to put some valuable inputs on how to get the point where this popup window flashes.

Thanks & Regards,

Swetha.