The action of intertwining operators on spherical vectors in the minimal principal series of a reductive symmetric space

We study the action of standard intertwining operators on H-fixed generalized vectors in the minimal principal series of a reductive symmetric space G/H of Harish-Chandra's class. The main result is that - after an appropriate normalization - this action is unitary for the unitary principal series. This is an extension of previous work under more restrictive hypotheses on G and H. The present result implies the Maass-Selberg relations for Eisenstein integrals of the minimal principal series. These play a fundamental role in the most-continuous part of the Plancherel decomposition for G/H.