  We investigated the mixed-stack charge-transfer crystal,
N,N'-dimethylphenazine-TCNQ (M$_2$P-TCNQ), which is polar at room temperature
and just at the neutral-to-ionic interface (ionicity $\rho \approx 0.5$). We
detect the typical dielectric signature of a relaxor ferroelectric and an
asymmetric positive-up-negative-down behavior. While relaxor ferroelectricity
is usually ascribed to disorder in the crystal, we find no evidence for
structural disorder in the investigated crystals. To elucidate the origin of
M$_2$P-TCNQ's dielectric properties we perform parallel structural and
spectroscopic measurements, associated with theoretical modeling and
quantum-mechanical calculations. Our combined effort points to a highly
polarizable electronic system that is strongly coupled to lattice vibrations.
The found indications for polarization reversal imply flipping of the bent
conformation of the M$_2$P molecule with an associated energy barrier of a few
tens of an eV, broadly consistent with an Arrhenius fit of the dielectric
relaxation times. While the polarization is mostly of electronic origin, its
possible reversal implies slow collective motions that are affected by
solid-state intermolecular interactions.
