Nonsurjective epimorphisms in decomposable varieties of groups

A full characterization of when a subgroup $H$ of a group $G$ in a varietal product ${\cal NQ}$ is epimorphically embedded in $G$ (in the variety ${\cal NQ}$) is given. From this, a result of S.~McKay is derived, which states that if ${\cal NQ}$ has instances of nonsurjective epimorphisms, then ${\cal N}$ also has instances of nonsurjective epimorphisms. Two partial converses to McKay's result are also given: when~$G$ is a finite nonabelian simple group; and when~$G$ is finite and ${\cal Q}$ is a product of varieties of nilpotent groups, each of which contains the infinite cyclic group.