It is known that superpotential couplings between messenger sector and MSSM matter fields can significantly change phenomenology of the standard GMSB models. Those interactions not only give rise to A-terms and soft masses but also can lead to large FCNC effects. In this talk, we shall discuss examples of SU(5) unification models in which messengers couple to all generations of MSSM matter and confront predictions of such scenarios with low-energy flavour observables. Emerging constraints on messengers couplings will be presented.