We investigate capillary condensation in slit-like pores with structured walls, described by a simple lattice gas model based on the mean-field approximation. Most theoretical workers did not go beyond very simple capillary models with perfect symmetry, thus being unable to map experiments in real pores with rather complex structure. In this paper we deal with chemical inhomogeneity, produced by a periodic wall potential, that causes fluid attraction and repulsion altering along the walls. We observe a new mechanism of capillary condensation, characterized by a splitting of the equilibrium "gas-liquid" transition, and we concentrate on the crucial role the typical length for the wall inhomogeneity, lambda, plays compared with the pore width, H. Our microscopic results are confirmed by a macroscopic analysis that leads to a modified Kelvin equation, giving a quantitative prediction of capillary condensation in structured pores. Furthermore our model explains the nature of the critical isochore as measured in mesopore condensation experiments by Thommes and Findenegg.