The goal of the paper is to derive two-sided bounds of the distance between the exact solution of the evolutionary reaction-diffusion problem with mixed Dirichlet-Robin boundary conditions and any function in the admissible energy space. The derivation is based upon special transformations of the integral identity that defines the generalized solution. To obtain estimates with easily computable local constants, the classical Poincaré inequalities and Poincaré type inequalities for functions with zero mean boundary traces are exploited. The corresponding constants were estimated earlier. Bounds of the distance to the exact solution contain only these constants associated with subdomains. It is proved that the bounds are equivalent to the energy norm of the error.