  The finite temperature QCD transition for physical quark masses is a
crossover. For smaller quark masses a first-order phase transition is expected.
Using Symanzik improved gauge and stout improved fermion action for 2+1 flavour
staggered QCD we give estimates/bounds for the phase line separating the
first-order region from the crossover one. The calculations are carried out on
two different lattice spacings. Our conclusion for the critical mass is $m_0
\lesssim 0.07 \cdot m_{phys}$ for $N_T=4$ and $m_0 \lesssim 0.12 \cdot
m_{phys}$ for $N_T=6$ lattices.
