I'm trying to solve a very simple heat conduction differential equation with NDSolve in Mathematica, but the solution I get is quite strange…
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This is a very simple one-dimensional solid-phase heat conduction differential equation, here is my code:

   

     a = NDSolve[{D[721.7013888888889` 0.009129691127380562` tes[t, x], 
         t] == 2.04988920646734`*^-6 D[tes[t, x], x, x], 
       tes[t, 0] == 298 + 200 t, tes[t, 0.01] == 298, 
       tes[0, x] == 298}, {tes[t, x]}, {t, 0, 0.005}, {x, 0, 0.01}]
    Plot3D[tes[t, x] /. a, {t, 0, 0.005}, {x, 0, 0.01}, PlotRange -> All]
    (Plot[(tes[t, x] /. a) /. t -> 0.0005, {x, 0, 0.01}, 
      PlotRange -> All])

After you run it, you will see: the temperature (in the equation it's named as tes) is lower than te0! It's ridiculous, it's against the second law of thermodynamics…how does this error come out? How can I correct it?
