In case you were asking about someone's breasts: they are real.
In case you were asking about a stock called NDN: don't invest at this point of time.
In case you were asking about the phenomena "NDN": they are coming for you.
In case you'ra a fanboy of the band "NDN":
Yes and no. They can be legitimately generated when an E-mail that you send does not get through the mailserver on the other end. HOWEVER, they are very often spoofed as a means of propagation for computer viri.