A direct and high-precision measurement of the mass difference of 123Te and 123Sb has been performed with the Penning-trap mass spectrometer SHIPTRAP using the recently introduced phase-imaging ion-cyclotron-resonance technique. The obtained mass difference is 51.912(67) keV/c2. Using the masses of the neutral ground states and the energy difference between the ionic states an effective half-life of 123Te has been estimated for various astrophysical conditions. A dramatic influence of the electron capture process on the decay properties of 123Te in hot stellar conditions has been discussed.