The boiler should not go to 210 Deg F. The temperature should not go much higher than 210 because the water will turn into steam and then you have essentially a potential bomb without the pressure relief valve.
Too many things to discuss here, first did you manually turn on the heat by overriding any limits or safeties, if so then you are more than likely ok as far as controls are concerned. If not then you may have a serious problem on your hands. The temperature control should not have allowed the temperature to go that high unless you have it set that high and there should be high limit to back up the temperature control set at around 200 and this should have shut it off also.
The pressure/temperature relief valve is doing it's job properly.
having said that there is several things that can affect the pressure in the boiler.
The first is the obvious one and that is the higher the temperature the higher the pressure as I had indicated above 210 deg F is too high and too close to steam.
The second is that you need to start at reasonable pressure to begin with, for instance, you have a cold water make up regulator that makes up water to maintain the pressure in the hot water pipe loop and subsequently the water in the boiler also. If this regulator is set too high or the spring has failed and the water gets cold normally the pressure will drop also and the regulator will add some water to get it to the set pressure on the regulator. Then when the water heats up the pressure gets too high. I have seen more water make regulators go bad than expansion tanks and they will make up too much water causing high pressure problems.
The third and last thing to check after everything above has been checked, including the temperature controls and safeties is the expansion tank.
There are two types of expansion tanks. A Diaphragm Tank or sometimes called a bladder tank and these are filled with compressed air or a steel holding tank, which traps air in the top half. The diaphragm tank is charged with air to the operating pressure of the boiler set by the regulating make up valve. Never release or add air to the diaphragm tank when it is till connected the boiler system. Tap on the tank and it should sound hollow through out. If the bladder has leaked the tank will completely fill with water and if not properly charged with air it will fill 1/2 way with water.
For the steel holding tanks there is sometimes a sight glass on the front of the tank to see the water level in the tank. The tank should be around half full. If the tank is filled near the top or completely filled you will need to drain some water. Turn the boiler off and close the valve that connects the expansion tank to the boiler and open the drain valve. Drain the tank until it is empty. Close the drain valve and the re-open the valve to the boiler. The expansion tank should filled to the half way mark or less and trap air in the top of the tank but if the tank fills again after a few days then there is a possibly a leak in your tank.
No residential boiler should ever have to heat water to 210 Deg F, 180 is about the maximum I have generally set commercial systems and that is during the coldest of winter months on old systems.