Scale-free topology of e-mail networks
  We study the topology of e-mail networks with e-mail addresses as nodes and e-mails as links using data from server log files. The resulting network exhibits a scale-free link distribution and pronounced small-world behavior, as observed in other social networks. These observations imply that the spreading of e-mail viruses is greatly facilitated in real e-mail networks compared to random architectures. 
Holger Ebel, Lutz-Ingo Mielsch, Stefan Bornholdt (Kiel University)