  A link stream is a collection of triplets $(t,u,v)$ indicating that an
interaction occurred between $u$ and $v$ at time $t$. Link streams model many
real-world situations like email exchanges between individuals, connections
between devices, and others. Much work is currently devoted to the
generalization of classical graph and network concepts to link streams. In this
paper, we generalize the existing notions of intra-community density and
inter-community density. We focus on emails exchanges in the Debian
mailing-list, and show that threads of emails, like communities in graphs, are
dense subsets loosely connected from a link stream perspective.
