  We study quantum non-Markovianity in the early stage of the emission process
of a two-level atom coupled to a semi-infinite waveguide, where the waveguide
termination behaves as a perfect mirror. Specifically, we restrict to the
analysis of the process for times shorter than twice the time delay t_d, where
t_d is the duration of a round trip along the atom-mirror path. We show the
emergence of a threshold in the parameters space separating the Markovian and
non-Markovian regions.
