  In this work, a diagrammatic representation of thermal mass effects is
derived from the $S$-matrix unitarity both in the classical and quantum
Boltzmann equations. Within the example of the seesaw type-I leptogenesis, we
discuss the connection of the Higgs thermal mass and the cancelations of
infrared divergences in zero- and finite-temperature calculations of the
right-handed neutrino decay and scattering processes.
