  A parabolic subalgebra $\mathfrak{p}$ of a complex semisimple Lie algebra
$\mathfrak{g}$ is called a parabolic subalgebra of abelian type if its
nilpotent radical is abelian. In this paper, we provide a complete
characterization of the parameters for scalar generalized Verma modules
attached to parabolic subalgebras of abelian type such that the modules are
reducible. The proofs use Jantzen's simplicity criterion, as well as the
Enright-Howe-Wallach classification of unitary highest weight modules.
