  We consider the case of very low reheating scenarios ($T_{\rm
RH}\sim\mathcal{O}({\rm MeV})$) with a better calculation of the production of
the relic neutrino background (with three-flavor oscillations). At 95%
confidence level, a lower bound on the reheating temperature $T_{\rm RH}>4.1$
MeV is obtained from Big Bang Nucleosynthesis, while $T_{\rm RH}>4.3$ MeV from
Planck data for very light ($\sum m_i = 0.06$ eV) neutrinos. If neutrino masses
are allowed to vary, Planck data yield $T_{\rm RH}>4.7$ MeV, the most stringent
bound on the reheating temperature to date. Neutrino masses as large as 1 eV
are possible for very low reheating temperatures.
