  The superconductor Sn_{1-x}In_{x}Te is a doped topological crystalline
insulator and has become important as a candidate topological superconductor,
but its superconducting phase diagram is poorly understood. By measuring about
50 samples of high-quality, vapor-grown single crystals, we found that the
dependence of the superconducting transition temperature Tc on the In content x
presents a qualitative change across the critical doping xc ~ 3.8%, at which a
structural phase transition takes place. Intriguingly, in the ferroelectric
rhombohedral phase below the critical doping, Tc is found to be strongly
ENHANCED with impurity scattering. It appears that the nature of electron
pairing changes across xc in Sn_{1-x}In_{x}Te.
