  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 transformation of the integral identity,
which defines the generalized solution, and exploits classical Poincare
inequalities and Poincare type inequalities for functions with zero mean
boundary traces. The corresponding constants are estimated due to Payne and
Weinberger, 1960, and Nazarov and Repin, 2013. To handle problems with complex
domains and mixed boundary conditions, domain decomposition is used. The
corresponding bounds of the distance to the exact solution, contain only
constants in local Poincare type inequalities associated with subdomains.
Moreover, it is proved that the bounds are equivalent to the energy norm of the
error.
