  Employing a recently proposed measure for quantum non-Markovianity, we carry
out a systematic study of the size of memory effects in the spin-boson model
for a large region of temperature and frequency cutoff parameters. The dynamics
of the open system is described utilizing a second-order time-convolutionless
master equation without the Markov or rotating wave approximations. While the
dynamics is found to be strongly non-Markovian for low temperatures and
cutoffs, in general, we observe a special regime favoring Markovian behavior.
This effect is explained as resulting from a resonance between the system's
transition frequency and the frequencies of the dominant environmental modes.
We further demonstrate that the corresponding Redfield equation is capable of
reproducing the characteristic features of the non-Markovian quantum behavior
of the model.
