Coulomb-blocked non-equilibrium transport through a single-level quantum dot at low temperatures is discussed. To calculate the occupation probabilities and the tunneling current including sequential tunneling, cotunneling and intrinsic spin-flip relaxation, we use a master equation approach based on the diagrammatic Keldysh formalism. The Coulomb diamond can be subdivided into parts differing in at least one of two respects: what kind of tunneling processes (i) determine the single-particle occupations and (ii) mainly contribute to the current. In the core and a shell region the single-particle occupations are determined by sequential and cotunneling, respectively. Therefore, no finite systematic expansions of the occupations and the current can be found that connects both regions. Alternatively, we construct a non-systematic solution, which is physically correct and perturbative in the whole cotunneling regime, while smoothly crossing-over between core and shell region. With this solution the impact of an intrinsic spin-flip relaxation on the transport is investigated. We focus on peaks in the differential conductance that mark the onset of cotunneling-mediated sequential transport and are located in the intermediate region between core and shell. It is shown, that these peaks are maximally pronounced at a relaxation roughly as fast as sequential tunneling.