  We consider a quantum-mechanical analysis of spontaneous emission in terms of
an effective two-level system with a vacuum decay rate $\Gamma_0$ and
transition angular frequency $\omega_A$. Our analysis is in principle exact,
even though presented as a numerical solution of the time-evolution including
memory effects. The results so obtained are confronted with previous
discussions in the literature. In terms of the {\it dimensionless} lifetime
$\tau = t\Gamma_0$ of spontaneous emission, we obtain deviations from
exponential decay of the form ${\cal O} (1/\tau)$ for the decay amplitude as
well as the previously obtained asymptotic behaviors of the form ${\cal O}
(1/\tau^2)$ or ${\cal O} (1/\tau \ln^2\tau)$ for $\tau \gg 1 $. The actual
asymptotic behavior depends on the adopted regularization procedure as well as
on the physical parameters at hand. We show that for any reasonable range of
$\tau$ and for a sufficiently large value of the required angular frequency
cut-off $\omega_c$ of the electro-magnetic fluctuations, i.e. $\omega_c \gg
\omega_A$, one obtains either a ${\cal O} (1/\tau)$ or a ${\cal O} (1/\tau^2)$
dependence. In the presence of physical boundaries, which can change the decay
rate with many orders of magnitude, the conclusions remains the same after a
suitable rescaling of parameters.
