  We study the properties of a refined weak coupling limit that preserves
complete positivity in order to describe non-Markovian dynamics in the
spin-boson model. With this tool, we show the system presents a rich and new
non-Markovian phenomenology. This implies a dynamical difference between
entanglement and coherence: the latter undergoes revivals whereas the former
not, despite the induced dynamics being fully incoherent. In addition, the
evolution presents "quasieternal" non-Markovianity, becoming non-divisible at
any time period where the system evolves qualitatively. Furthermore, the method
allows for an exact derivation of a master equation that accounts for a
reversible energy exchange between system and environment. Specifically, this
is obtained in the form of a time-dependent Lamb shift term.
