Consider a system contained in a very tall adiabatically isolating vessel with rigid walls initially containing a thermally heterogeneous distribution of material, left for a long time under the influence of a steady gravitational field, along its tall dimension, due to an outside body such as the earth. It will settle to a state of uniform temperature throughout, though not of uniform pressure or density. Pressure and density will be higher in the lower part of the vessel due to gravity. Is this system in mechanical equilibrium? I mean, we don't have macroscopic change of the pressure in the system once everything is settled down, but pressure is not spatially uniform and it depends on the height we measure it.