  Confirmed by the measurement of neutrino oscillations, neutrino mass is
recognized as a genuine manifestation of physics beyond the Standard Model,
while its originating mechanism remains a mystery. Moreover, the proper
field-theory description of neutrinos, whether they are Majorana or Dirac type,
must be linked to such a mechanism. The present work addresses the calculation,
estimation, and analysis of one-loop contributions from virtual Majorana
neutrinos, light and heavy as well, to the neutral gauge boson coupling $ZZZ$,
which participates in $Z$-boson pair production from $e^+e^-$ collisions. This
task is carried out in the framework defined by a seesaw variant in which light
neutrinos remain massless at tree level, then becoming massive radiatively. The
$ZZZ^*$ coupling, with $Z^*$ an off-shell $Z$ boson, is defined by two form
factors, namely, $f_4$, characterizing CP-odd effects, and $f_5$, which is
CP-even. Constraints from the Large Hadron Collider on both these quantities
are currently ${\cal O}(10^{-4})$. Our calculation yields CP-nonpreserving
contributions to $ZZZ$, which are absent in the framework of the sole Standard
Model. Our estimations show that the $f_4$ contribution might be as large as
${\cal O}(10^{-7})$ for heavy-neutrino masses $\sim1\,{\rm TeV}$. CP-even
contributions $f_5$ are also generated, which are, in general, larger than
their CP-odd counterparts. We estimate them to be as large as ${\cal
O}(10^{-4})$ at a center-of-mass energy of $500\,{\rm GeV}$, in $e^+e^-$
collisions.
