Anything above 32deg F/0 deg C _should_ be well and safe.
Feel free to stop reading before this line.. Science Content Follows
There is a polymer between the anode and cathode of the cell. It's what keeps them from shorting out. Thermal Runaway (aka fire/explosive fire) is caused by a breakdown in the polymer. This can happen
through jarring or over charging (causing too much expansion in the cathode), heating, or cooling. Over discharging causes contraction which has the same effect.
Anywho, freezing them then jarring them can crack the polymer (or these days, thanks to Matsusita, the polymer/ceramic...) and introduce the anode and cathode. The jarring can be mechanical, or it can be electrical (ie. putting charge-power to the cell)
Any tiny hairline fractures can, over time, spread causing the short. As the density goes up (by introducing more electro-chemical component), the insulation goes down [predictably]. So as battery demands progress the risks go up. There is technology to help limit this (such as the non-conductive metallic ceramic components), but it's still a tremendous risk.
So the upshot is: Never freeze a Li* battery if you can help it. And if you can't help it (such as in the mail box), then be as gentle (mechanically and electrically) with it as possible, letting it slowly come back to equilibrium (room temp).