UHE cosmic neutrino interaction with the cosmic neutrino background (CνB) is expected to produce absorption dips in the UHE neutrino flux at energies above the threshold for Z-boson resonant production. The observation of these dips would constitute an evidence for the existence of the CnuB; they could also be used to determine the value of the relic neutrino masses as well as some features of the population of UHE neutrino sources. After breafly discussing the current prospects for relic neutrino spectroscopy, we present a calculation of the UHE neutrino transmission probability based on finite-temperature field theory which takes into account the thermal motion of the relic neutrinos. We then compare our results with the approximate expressions existing in the literature and discuss the influence of thermal effects on the absorption dips in the context of realistic UHE neutrino fluxes and favoured neutrino mass schemes.