  We have shown in a preceeding paper how to normalize intertwining operators
for classical groups using the twisted endoscopy lifting. In this paper, we
prove that the image of such an operator in the cases interesting in the theory
of Eisenstein Series, is either 0 or an irreducible representation. As a
consequence we compute explicitly the points where Eisenstein Series for square
integrable representations are not holomorphic under some hypothesis at the
archimedean places: at that places we mainly assume that the infinitesimal
character is integral and regular.
