ambrus,
The user isn't the one sending the message, the system would be. Presumably, spam detection and throttling would be part of a solution to prevent this sort of thing. Perhaps only those who have selected "notify me when someone links to one of my nodes" in their user settings would get such messages.

It is important to flesh out the technical hurdles. I am more interested what the gods and devils have to say with regards to the chances of something like this happening at all.