This paper deals with the problem of Disturbance Rejection by Dynamic Output Feedback. It aims at proposing new geometric and structural characterizations for the Fixed Poles of this problem, i.e. the greatest set of fixed poles that are present in the closed loop system for all possible solution (whatever be the way used to find this solution). We also propose minimal solutions to the problem in terms of the fixed poles, i.e., solutions for which all the poles are freely placed, except the fixed poles of the problem. As a corollary, we propose a structural necessary and sufficient condition for the existence of internally stable solutions, which generalizes the sufficient structural condition proposed by Basile and Marro. It has to be noted that no restrictive assumption is made on the system concerning controllability or observability. Our results are established under the natural and unrestrictive assumption that the overall state description is minimal, i.e. controllable and observable, but with respect to all the inputs (control and disturbance) and all the outputs (controlled output and measurement).